Abstract: Atlantic salmon Salmo salar L. juveniles with various initial body weight of (30.25±2.12) g (SW1 group), (24.27g±1.59)g (SW2 group), (18.05±2.46)g (SW3 group) were reared in a recirculating system under a photoperiod of 12:12 light: dark in 1-42 days and 24 light: 00 dark in 43-84 days to evaluate the effects of size on growth performance and mechanism of physiological response during smoltification. The results showed that the maximal condition factor, daily weight gain, food conversion efficiency, and net yield were found in SW2, with significant difference in different experimental groups (P<0.05). There were significantly higher special growth rate in body length and body weight in SW2 than in other groups (P<0.05); and there were significant differences in coefficient of body length and body weight variation among the three groups (P<0.05). The CV of body length of S. salar in the different groups did not significantly change in the first 6 weeks of the study. But when the condition was changed to full light the coefficient of variation of body length significantly increased in the SW1 and SW2 groups, and the change was more significant in group SW1 compared to the other groups (P<0.05). The CV of body weight of S. salar in group SW3 varied greatly over the course of the experiment. Within the first 6-week light cycle (12L:12D), the coefficient of variation of body weight in SW1, SW2 showed insignificant difference (P>0.05). After the condition was changed to full light in the last 6 weeks, the coefficient of variation of body weight in 3 groups was narrowly varied. At 84 d, the value in the SW3 group was significantly higher than that in any other group (P<0.05). The hemoglobin concentration and the number of red blood cell (RBC) were found to be increased with the increase in size, indicating that the increase demand of oxygen for fish. The maximal white blood cell (WBC), lymphocyte, neutrophils was found in SW3 group, which showed that enhanced their disease-resistant capability. The number of red blood cells did not differ significantly between groups SW1 and SW2 (P>0.05), but the value was significantly higher than that in group SW3 (P<0.05). The number of thrombocytes fell with the increase of culture size, and the three groups differed significantly (P<0.05).The number of neutrophil was close among these groups, and no significant difference was identified (P>0.05). The highest values of total protein (TP) and albumin (ALB) appeared in SW2 group, and the difference of three groups were marked (P<0.05). The concentration of total cholesterol (TC) and triglycerides (TG) increased with the increase in size (P<0.05), which suggested that the enhancement of energy utilization in the blood and body. The concentration of uric acid and carbamide in metabolites was highest in group SW2. Their concentrations in groups SW1 and SW2 did not differ significantly (P>0.05), and they were significantly higher than those in group SW3 (P<0.05). The concentration of T-Bil in group SW3 was significantly higher than those in the other two groups (P<0.05).The concentration of glucose were no significant difference (P>0.05), which suggests that utilization rate of glucose by Salmo salar L. is lower. The concentration of CK was highest in group SW3, but no significant difference was identified among the three groups (P>0.05). So Salmo salar L. in 24.27±1.59g should be selected for the smoltification, thereby optimizing the culture condition and increasing the culture yield per water body at lower cost, which can theoretically referenced for the scale aquaculture of Salmo salar L. seeds.