gillnet in the South China Sea, fishing trials for gillnets with five different mesh sizes (40, 45, 50, 55 and 60 mm with twine diameters of 0.25, 0.25, 0.30, 0.30 and 0.35 mm, respectively) were carried out in the northern of South China Sea in May 2008. The catches were composed of 72 species in all, of which only 13 species were occurred in all experimental gillnets. Based on the species, abundance and biomass of catch, the methods of diversity index, similarity index and abundance-biomass comparison(ABC) curve was used in the study of intra-selection of gillnet and the effect of different mesh size gillnets on fishery resources. The results indicate that the catch species composition were different between the experimental gillnets. With the mesh size , the species and abundance of catch have increased, the average individual weight decreased. Trichiurus haumela and were the dominant catch species, biomass percentage of all catches was 21.35 %, 15.76 %, 14.78 % ,8.43 % and 7.20 %, have higher of percentage 40mm and 45mm gillnets, 50mm, 55mm and 60mm gillnets. The stability of biological community of catches have downward trend with mesh size reduced. Statistical value of 40, 45, 50, 55 and 60 mm gillnets was -0.023 , -0.012, 0.039, 0.083 and 0.073 respectively, 40 and 45 mm gillnets have a serious effect on fisheries resources. were the targeted species in the fishery. Minimum mesh size of should be d based on the size selection and the minimum fishing size regulation of the three species,the effect of twine diameters on fishing power of gillnets should also take into account.