Abstract: Based on mesh selectivity data of double bottom trawl codend tested in the offshore waters of northern South China Sea in October 1997 and from November to December 2000, catch selectivity of square mesh codends between 35.2 and 30.3 mm of trawl was compared and analysed. The results are as follows. (1)Because the total escape rates of 35.2 mm square-mesh codend for catch (39.28% in number and 16.18% in weight) are higher than that of 30.3mm square-mesh codend for catch (28.40% in number and 7.47% in weight), catch selectivity of 35.2 mm square mesh codend is slightly better than that of 30.3 mm square mesh codend.(2)The main species in 35.2 mm square mesh codend and its cover net are mostly the same as 30.3mm square mesh codend and its cover net, and there are lesser and fewer in commercial species and its numbers in their cover nets. (3)The escape rates of Trachinocephalus myops, Upeneus bensasi and Saurida tumbil as dominant species released are 42.9%~65.8% in number and 24.2%~45.0% in weight respectively in 35.2 and 30.3 mm square-mesh codends, and the 50% selection lengths (L_0.5) caught S. tumbil and U. bensasi are smaller than their minimum fishable size. It is considered that there are no distinct difference on catch selectivity of square mesh codends between 35.2 and 30.3 mm of trawl and satisfactory and responsible fishing can be achieved no other than enlarging mesh size of the codend of bottom trawl based on the comprehensive analysis.