More or less at the center of the clusters of islands off Trang's coast is the island known either as Koh Hai or Koh Ngai. Technically, the island is in Krabi Province, but it's more commonly reached through Trang, so it's included here.
Map of Trang Islands
Koh Ngai is just 5 kilometers square, and mostly covered by jungle surrounded by a fringe of white sand beaches. A coral reef runs down the eastern side of the island, which is where most of the resorts are located.
The island is just 16 kilometers from the mainland pier at Pakmeng in Trang, as well as just 18 kilometers from Koh Lanta Yai. It's also just 8 kilometers from the island of Koh Mook with it's fantastic Emerald Cave.