A total of 782 sea slug species (opisthobranch molluscs) within 161 genera, 56 families, and 7 orders are authentically recorded from Taiwan¡¦s Exclusive Economic Zone. Among these species, 459 species (58.7 %) are identified to species level and the other 323 species (41.3 %) are undescribed species. In terms of the whole Opisthobranchia fauna of the tropical western Pacific Ocean, approximately 1000 species are expected from Taiwan¡¦s EEZ. As most of the previous surveys on within-area species diversity were carried out in daytime, diel variation (i.e., day and night) in the species composition of sea slugs has been neglected. In order to assess whether such estimations for community diversity based only on daytime surveys are accurate, I undertook replicated diel surveys for 12 consecutive months (from December 2009 to November 2010) at Shilang Marine Reserve, Green Island (Lyudao), Taiwan. Phyllidiella pustulosa was observed most often during the daytime surveys, while Tritonia sp. 1 (an undescribed species) was the most abundant species at night. The results showed the species composition was clear different. During the field surveys, I found the difficult to identify the phyllidiid nudibranchs. Hence, I search for the additional taxonomic characters for phyllidiid nudibranchs base on integumentary spicules and gene sequences (COI and 16S genes). The results showed the spicules characters might provide the character to distinguish the Phyllidiella from other phyllidiid genera, but they were not informative at the species level. The results on molecular phylogeny showed that taxonomic status (both at the species and generic levels) of most phyllidiid genera, except Phyllidiella, received strong supports from COI gene and 16S gene. In contrast, each Phyllidiella species did not form a clade (suggesting the possibility of over estimating the species number in this genus). Finally, I described and illustrated three distinctive new species of Tambja (Nudibranchia: Polyceridae) from Taiwan and Australia. Among these three species, two of them were collected from Australia the other was collected from Taiwan. Tambja dracomus sp. nov. which is only distribution in cool temperate coastal waters in southeastern Australia and northern New Zealand; T. caeruleocirrus sp. nov. which is distributed in warm temperate coastal waters in eastern Australia, southeast Pacific Ocean. Tambja pulcherrima sp. nov. is widely distributed throughout the tropical, subtropical and warm temperate waters in western Pacific Ocean from Japan to northern New Zealand.