A field experiment of different density of three shellfish ages was carried out in the sea of raft culture of Pationopecten yessoensis in Changhai County from August 2009 to July 2010 to study the self-thinning effect and to estimate the optimal culture density using the monthly data of the growth of shell length,wet weight growth and cumulative mortality.No statistical difference was found in the anniversary shell length growth(P>0.05)in different density for the surveyed scallop samples of every shellfish age.The shell length of the first-year scallop grew fastest with an average annual growth 29.53 mm,followed by 23.50 mm for the second-year shell growth and 15.47 mm for the third-year shell.The anniversary of wet weight growth was not significantly different(P>0.05)either,the wet weight growth of the first-year scallop,the second-year and the third-year scallop were 14.69,37.80 and 54.07 g,respectively.The difference in cumulative mortality of different density groups for second-year and third-year shellfish was not statistically significant(P>0.05) with the third-year shellfish mortality ranging from 76.30% to 83.33% and the second-year shellfish mortality rate ranging from 88.89% to 91.78%.The cumulative mortality of the first-year scallop with cultural density of 150 ind/layer was significantly different(P <0.05)from that of the density of 80 ind/layer and 50 ind/layer.Based on self-thinning model,the suitable raft breeding density is 4-5 ind/layer in third-year scallop,8-16 ind/layer in second-year,29-124 ind/layer in first-year,respectively.