The dominant tree species here are; Terminalia catappa, 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 are found. These mangrove forests are tree species of Rhizophora mucronata, R. apiculata, Ceriops decandra, C. tagal, Bruguiera gymnorrhiza, B. parviflora, Avicennia alba, Xylocarpus granatum, X. moluccensis, Hibiscus tiliaceus and Cerbera odollum. High biodiversity of marine habitats can be found in the area including rocky shores, sandy beaches, seagrass beds and coral reefs. Fringing reefs develop around most of the islands and coral communities are also found on off-shore pinnacles.
The total coral reef areas are approximately 16 km2 and over 130 scleractinian coral species have been reported with average live coral coverage around 40% . Coral reef conditions around Koh Chang and small islands nearby, except the northern part, are poor. The dominant corals are Porites lutea, Pavona decussata, Echinopora lamellosa, Goniopora spp., Pavona spp., Symphyllia spp., Fungia spp. and Astreopora sp.
The sponge, Xetospongia sp. is also abundant. Coral reef conditions of a group of small islands north of Koh Chang are fair with an abundance of Porites lutea and Symphyllia spp. Coral reef conditions of the Koh Mak group, the largest coral reef area, are poor. The dominant corals are Porites lutea, Diploastrea heliopora, Goniopora spp. and Symphyllia spp. The sponge Xetospongia sp., soft coral Sinularia sp., giant clam Tridacna spp. are also common in the area. Coral reef conditions around Koh Kut groups are fair with an abundance of coral Porites lutea, Diploastrea heliopora, Symphyllia spp., and Faviidae, and giant clams Tridacna spp. The severe coral reef bleaching phenomenon in 1998 resulted in coral reef degradation of Mu Koh Chang, however natural recovery is observed. Coral reef fishes of Mu Koh Chang are comparatively high in both abundance and diversity.
Over 113 species of fishes are recorded, comprising economically important species such as Lutjanidae, Serranidae and Haemulidae, coral reef indicator species such as Chaetodontidae and common coral reef fishes such as Apogonidae, Labridae and Pomacentridae. Several endangered species, such as sea cows, dolphins, whales, sea turtles (Chelonia mydas and Eretmochelys imbricata) and whalesharks (Rhincodon typus) can be found in the area. In general, the marine organisms found at Mu Koh Chang are of relatively high diversity for the western section of the South China Sea.