The Khao Laem National Park is located in the folded complex mountains of the Tanaosi Mountain range. It runs north to south and is between elevations of 100 to 1,800 meters above sea level. This area is predominantly composed of limestone, sandstone, and shale, with the highest summit, Khao Khiao-Khao Yai (Khao San Nok Wua Summit), standing at 1,767 meters above sea level. 

         The combination of a high complex mountain landscape terrain results in the formation of numerous rivers and waterways that flow into Khwae Noi River and Khao Laem Reservoir. These include Ranti River, Pom Pi Nai Creek, Bikhi Yai River, Ong Phra Creek, Kriang Krai Creek, Pilok Creek, Pracham Mai Creek, Luek Creek, Nam Sap Creek, Kaeng Kha Yue Creek, Patong Creek, Song Kalia Creek, Tiphi Creek, Thi Mong Tha Creek, Wang Khayai Creek, Chong Khaep Creek, Tha Maduea Creek, Nam Mut Creek, and Kroeng Krawia Creek.  The reservoir above Khao Laem Dam built in 1979 by Thailand's Electricity Generating Authority (EGAT) started retaining water in 1984. The normal water retention level of the reservoir is 155 meters, with a maximum of 160.5 meters above sea level. The water surface area of Khao Laem National Park is approximately 388 square kilometers, or 25.92% of the National Park's total area.

      The climate is tropical, influenced by the southeasterly winds and Andaman Sea breezes. It has three seasons: the rainy season lasts from mid-May to October, with the heaviest rainfall occurring in July and August. Annual rainfall averages between 1,600 and 2,200 millimeters, with the highest totals in the National Park's northernmost section. The winter months (between November and February), the lowest average temperature is 14 degrees Celsius in December. Summer, from March to mid-May, the maximum average temperature is 37 - 38 degrees Celsius in April.

       The forest in Khao Laem National Park is classified as a mixed deciduous forest with many different plant species and thick canopy. In some areas with high humidity, such as around or along the creek, there are especially dense trees and the conditions are similar to tropical rainforests. Most of the forest area is in the east and west of the National Park including plant species such as Lagerstroemia loudonii, Stereospermum fimbriatum, Iron Wood, Beleric myrobalan, Phoebe paniculata, Vitex peduncularis, Croton persimilis, Microcos paniculata, Jambolan Plum, Pakia sumatrana, Upper Wood Grain, Caryota urens, Ixora cibdela, Climbing Wattle, and Aporosa villosa. 

        Along with the mixed deciduous forest, there are degraded forest areas, which include encroached areas that have been converted to agricultural land, most commonly in the east of the National Park around a reservoir and along the National Park's southeastern boundary. The encroached and degraded land was later left abandoned and replaced by grass, forest fruit plants, and bamboo. Plants such as Croton oblongifolius, Java Plum, Hymenodictyon orixense, and Korlan (Nephelium hypoleucum) are discovered.

       Along with the mixed deciduous forest, there are degraded forest areas, which include encroached areas that have been converted to agricultural land, most commonly in the east of the National Park around a reservoir and along the National Park's southeastern boundary. The encroached and degraded land was later left abandoned and replaced by grass, forest fruit plants, and bamboo. Plants such as Croton oblongifolius, Java Plum, Hymenodictyon orixense, and Korlan (Nephelium hypoleucum) are discovered.

There are at least 268 species of wild animals living together in Khao Laem National Park. Commonly found species include Barking Deer, Lar Gibbon, Common Palm Civet, Asian Small-Clawed Otter, Black Giant Squirrel, Callosciurus caniceps, Indochinese ground squirrel , Greater Short-Nosed Fruit Bat, Hill Long-Tongued Fruit Bat, and Dawn Bat.

     Birds found include Puff-Throated Babbler, White-Crested Laughing Thrush, Gampsorhynchus torquatus, Black-Crested Bulbul, Stripe-Throated Bulbul, Black Bulbul, Greater Flameback, Laced Woodpecker, Heart-Spotted Woodpecker, Collared Falconet, Raffles' Malkoha, Red-Headed Trogon and Red-Throated Barbet. There are also reptiles and amphibians such as the Eutropis macularia, Spotted Forest Skink, Orange-Winged Flying Lizard, Calotes versicolor, Calotes emma, Tokay Gecko, Banded Krait, Red-Necked Keelback, Green Paddy Frog, Dark-Sided Frog, Rice Field Frog, Noisy Chorus Frog, Twin-Spotted Bushfrog, Mottle Bush Frog, Common Asian Toads, Asiatic Burrowing Frog, Dark-sided chorus frog, Inornate Chorus Frog, and Ornate Narrow-Mouthed Frog. Around Khao Laem Dam and streams, there are various freshwater fish such as Grey Featherback, Oxygaster anomalura, Scissor-Tailed Rasbora, Jullian's Mud Carp, Beardless Barb, Black-Spotted Long Tom, Giant Snakehead, Discherodontus halei, Crossocheilus oblongus, Hillstream Loach, Pangio anguillaris, Siamese Rock Catfish, and Bagrid Catfish.