Coral reef is one of the prominent habitats in Taiwan where sea slugs are common inhabitants. However, research on sea slug in Taiwan still remains deficient. Between June 2004 and May 2006, surveys on the western coast of the Kengting Peninsula, the northeastern coast of Taiwan, Lan Yu, Green Island, Liuchiu Island, and Penghu were conducted for sea slugs. In total, 4 suborders, 14 families, and 105 species were recorded; 85 species were new records, and 6 were possible new species. Among these 105 species, specimens of 44 species in 8 families were available for investigation of spicule morphology which has been considered as an uninformative taxonomic character. Five types of spicules were recognizable in which the spindle-like type had broadest distribution among the species and is considered as the plesiomorphic character state. A possible autapomorphic type was found in Phyllidiella pustulosa; this status can be ensured until spicules of other congeners are known. Distribution of the other two spciule types provided information for a hypothesis for the family phylogenetic interrelationship which shows certain agreement with existing phylogenetic hypotheses. In Dorididae and Chromordorididae, distribution of the spicule types showed no spatial discrepancy in the whole animal, whereas such discrepancy pertained in Phyllidiidae. More information on spcicule morphology are needed before their taxonomic importance can be justified.