In order to reduce the catch rate of immature fish caught by baby bottom trawler in Yilan bay, this study was to conduct a mesh selectivity experiment with the cover net method during November 2 - December 16, 2004. The mesh size of trawl net at cod end was 7 cm and its outside attached cover net was 3 cm. There are 14 species of fishes and 6 species of shrimps were measured by their selection span which based on their linear biomass were equal and greater 0.5 with a significant difference less than 5% in logistic regression analysis. The fishing efficiency of mesh size of 7 cm and the selection span was differed with the ratio of body width to total length. There were 67.55% of Japanese jack mackerel (Trachurus japonicus) escaped from cod net into cover net. The escape rate for other fishes, Japanese butterfish (Psenopsis anomala), Red bigeye (Priacanthus macracanthus), Largehead hairtail (Trichurus lepturus), Blackmouth croaker (Atrobucca nibe) and Brushtooth lizardfish (Saurida undosquamis) was 38.20, 100.00, 54.60, 35.71 and 54.43%, respectively.