The mortality of the Pohai Sea prawns in autumn has been estimated with avariety of methods. After mating, male prawns die in large numbers. We have found that undersome assumptive conditions the mating mortality of male prawns may be estimated withsex ratio. The results are Iisted in table 2. The mating mortality is about 0.65 in lateOctober. The accumulative value of mating mortality of male prawns is about 1.05 inesrly December. The total mortality of Pohai Sea prawns have been estimated. The results are asfollows. The catch in number per month and catchpereffort as indices of abundanceare regressed against time to calculate the total mortlity. For ten-day average in 10years is about 0.25 for femals 0.34 for mals, and that in 12 ysars about 0.23 forfemals 0.34 for mals. Now we assume that the natural mortality of female prawns maybe negligible in comparison with its fishing mortality, then the total mortality of femaleprawns is approximately equal to its fishing mortality which is about 0.25 per ten-dayperiod in about 800 standard unit of fishing effort. Therefore the catchability is equalto 3.125×10^-4. After discussing the relationship between mortality and fishing effort, we confirmthat when fishing effort is less than 1000, fishing mortality varies with effort, andwhen fishing effort is greater than 1000, the increase in effort has little influence onfishing mortality. The fact shows that there is a certain limit, being about 0.3perten-day period to fishing mortality with regard to the prawn fishery in the autumnfishing season.