Abstract: Based on mesh selectivity data of trawl codend tested in the offshore waters of Northern South China Sea in October 1997 and in November 2000,catch selectivity of 30.3 mm square mesh codend was analysed. The results indicated that the average escape rates of 30.3 mm square mesh codend for catch were 29.76% in number and 8.46% in weight. There were lesser and fewer in commercial species and its numbers in the cover net, and the sum of Nemipterus virgatus,Saurida tumbil,Upeneus bensasi,Trachinocephalus myops comprised 95.4% in number and 86.0% in weight of sample catch in the cover net,respectively. The high percentage of juvenile commercial species that were undersized in the codend showed 30.3 mm square mesh codend had more serious damage to most juvenile commercial species, approaching or exceeding 50% in percentage of juvenile species which body length is smaller than fishable sizeTrichiurus haumela, Priacanthus tayenus, Navodon tessellatus, Argyrosomus argentatus, Nemipterus bathybius and so on, even exceeding 90% in percentage of juvenile Sepia spp, Loligo spp and Parastromateus niger. In particular, 50% selection lengths (L0.5) caught Saurida tumbil and Nemipterus virgatus were smaller than their minimum fishable size. To sum up, the use of 30.3 mm square mesh codend will make against sustainable utilization to most fishery resource and biodiversity protection in Northern South China Sea.