By plane: From Bangkok you can get on a plane to Trat. It will take 45 minutes. Then by ferry you’ll reach the island.

View on Koh Chang from the Trat-Koh Chang ferry
By bus: from Bangkok (airport or bus terminal) – 4 hours; from Pataya – 3 hours.
However your are planning your way to the island, you will eventually end up taking the ferry from the Trat city.
By plane: From Bangkok you can get on a plane to Trat. It will take 45 minutes. Then by ferry you’ll reach the island.
View on Koh Chang from the Trat-Koh Chang ferry
By bus: from Bangkok (airport or bus terminal) – 4 hours; from Pataya – 3 hours.