The mountain slopes are covered by dense Tropical Evergreen Forest which due to the historically low level of habitation on the island is largely undisturbed (until recently when more people move here) The principle tree species present include Dipterocarpus alatus, D. turbinatus, Anisoptera costata, Hopea odorata, Irvingia malayana, Podocarpus neriifolius, Diospyrosspp., Castanopsis spp., Croton spp., Oncosperma horrida, Caryota mitis., Daemonorops spp., Korthalsia grandis, Bauhinia bracteata, Freycinetia sumatrana, Platycerium coronarium, Amomum spp., Boesenbergia pandurata and Kaempferia pulchra. Around the villages of Salak Phet, Salak Khok, Khlong Son and Khlong Phrao Bay are areas of Beach Forest. The dominant tree species here are Terminalia catappa, Calophyllum inophyllum, Melaleuca leucadendra, Eugenia grandis, E. spicata, and Pandanus odoratissimus. In the more sheltered spots, especially where freshwater enters the sea to produce brackish water, fairly large areas of Mangrove Forest exist. These mangrove forests consists of a combination of tree species including Rhizophora mucronata, R. apiculata, Ceriops decandra, C. tagal, Bruguiera gymnorrhiza, B. parviflora, Avicennia alba, Xylocarpus granatum, X. moluccensis, Hibiscus tiliaceus and Cerbera odollum.
Wildlife surveys undertaken in 1992 recorded 29 species present within the national park. Animals seen included; Wild pig, Barking deer, Slow loris, Stump-tailed macaque, Silvered langur, Eastern mole, Small Indian civet, Javan mongoose plus several species of Bats, Squirrels, Rats and Mice. The bird list for the park currently numbers 74 species of which 61 are resident. More common sightings include Pacific reef-egret,
Yellow-vented bulbul, Nightjar, Pacific swallow, White-breasted waterhen, Green imperial pigeon and Oriental pied hornbill. Migratory visitors
to the islands include Little heron, Greater sand-plover, White-winged tern, Brown noddy, Black-naped oriole, Pale-legged leaf warbler, Artic warbler and Barn swallow. Two common breeding visitors to the islands are the Blue-winged pitta and the Hooded pitta, both of which have very distinctive whistling calls often heard early in the morning and in the evening. Commonly seen passage migrants include Blue-throated flycatcher, Blue-and-white flycatcher and the Eastern-crowned warbler. The survey team discovered 42 species of reptiles and amphibians.
More commonly seen of which are; Malayan mud turtle, Indian monitor lizard, Water monitor lizard, White-face blue garden lizard, Tokay gecko, East indian brown sided grass skink, Python, Racer, Rat snake, King cobra, Common asiatic toad, Green ricefield frog, Common tree frog, Bulterís chorus frog and Rugosed frog. An endemic species found here and nowhere else is the aptly named Kohchang frog ( Rana kohchangae).
Ko Chang islands are situated a long way away from any mainland rivers and thus the water around the islands is very clear and free from sediment. Thus the beaches are of clean white sand with crystal clear seawater and rich marine life. In the sheltered locations coral reefs occur with numerous coral types such as Massive coral, Staghorn coral, Laminar coral, Columna coral, Giant clams, Sea anemone, Sea fan, Sea star, Marine fishes and seaweeds. This marine diversity can be observed at many locations around the park, particularly good sites include Ko Chang noi, Ko Yuak, Ko Man Nok, Ko Khlum, Ko Wai, Ko Lao Ya, Ko Ngam, Ko Rang, Ko Kra, and along the eastern side of Ko Chang island. Another good place to explore the marine life is around the wrecks of the Thai warships "Songkhla" and "Chonburi" in Salak Phet bay on the southern end of the island.