The biodiversity of the Trichoptera fauna was studied at 7 waterfalls on 6 islands within Thai Gulf and Andaman Sea, Thailand. Sixty-five species, 26 genera and 12 families of caddisflies were found. The dominant caddisfly families on the islands were Hydropsychidae, Philipotamidae, and Leptoceridae. Cheumatopsyche chrysothemis Malicky and Chantaramongkol, 1997, and Chimarra khamuorum Chantaramongkol and Malicky, 1989, were the most abundant species found on the islands. Occurrence of Trichoptera species on the islands and in neighboring areas is discussed.