Abstract: Based on experiments done in Tokyo Bay from 1995 to 2000，the results show the mesh size selectivity of pots with mesh size 21.0,18.1,15.5,13.6,11.6 cm is nearly zero as 0l25,50 cm, 0l22.50 cm, 0l18.50 cm, 0l18.50 cm, 0l16.50. The selectivity increases as mesh size gets larger as 25.50l35.50 cm, 22.50l32.50 cm, 18.50l27.50 cm, 18.50l26.50 cm, 16.50l24.00 cm. As l＞35.50 cm, l＞32.50 cm, l＞27.50 cm, l＞26.50 cm,l＞24.00 cm, the selectivity gets to 1.00. By the theory of AIC, the influence of cover net is notability when the mesh size of cover net is close to that of experimental net and single curve model fits better. The 50% selectivity of G/Mc is 1.11, the selectivity range is 0.52. As 0G/Mc 0.55, selectivity is nearly to zero and as 0.55G/Mc 1.60, selectivity increases as G/Mc gets larger. As G/Mc ＞1.60，the selectivity is 1.00.