The relationship between sample size and the accuracy in the assays on immune activity of population of Fenneropenaeus chinensis was studied. The superoxide dismutase （SOD） activity, lysozyme （LSZ） activity and the total haemocyte counts （THCs） of the juvenile shrimp were detected with xanthine-oxidation method, advanced Hultmark method, and haemocytometer method, respectively. The mean coefficient of variation of the three assays were ? 2.34%, ?4.67% and ? 4.58% respectively. When a predefined confidence zone of the sample＇s observation located in the range of acceptable error, the observation on samples reflected the real state of the population. When the mean coefficient of variation of the assays was regarded as the acceptable errors of the detections, and a 95% confidence level of the observation was adopted, the sample sizes of 80, 70 and 120 were needed for the accurate reflection of the real SOD, LSZ and THCs of the population respectively. If a 90% confidence level of the observation was adopted, the sample sizes for the reflection of real SOD, LSZ and THCs of the shrimp population were as large as 50, 50, 80 resoectively.