The rock of the island is mainly Granitic and dates from approximately 230-195m years ago. The geology produces a very mountainous is land, but with rounded slopes rather than sheer cliffs.

The island of Koh Chang has many streams and rivers which are fed by the rainfall collected on the mountainous slopes of the island. Due to the great amount of rainfall here these streams are very clean and supply water all year round. The principle streams are Khlong Mayom, Khlong Son, Khangkhao, Khlong Bang Bao, Khlong Phrao, Khlong Nonsi, and Khlong Chaiyachet. This mix of mountains and streams produces some very beautiful waterfalls the most notable are: Than Mayom, Khlong Phlu, Khlong Nonsi, Khiri Phet and Khlong Nung Waterfalls.

The islands are situated a long way away from any mainland rivers and thus the water around the islands isvery 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, Stagshorn coral, Laminar coral, Columna coral, Gaint clams, Sea anemone, Sea fan, Sea star, Marine fishes and seaweeds. This marine diversity can be observed at many locations around the park, paticularly good sites include: Koh Chang noi, Koh Yuak, Koh Man Nok, Koh Khlum, Koh Wai, Koh Lao Ya, Koh Ngam, Ko Rang, Koh Kra, and along the easternside of Koh 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.