The influence of temperature fluctuation on physiological parameters and immune activity of three different sized scallop (Patinopecten yessoensis) was studied in this work. Temperature fluctuated in the range of 15-10-15℃, with increasing or decreasing 5℃ per 2 h. Temperature was totally fluctuated four times in the experiment, which named as B1, B2, B3 and B4. Results showed that the mortality rates of big, medium and small sized P. yessoensis were 4%, 6% and 6%, respectively. The mortality rate of big sized P. yessoensis was relatively lower, and the mortality only appeared in B1 and B2. Oxygen consumption rates of three different sized scallop were lower at B1 than the initial value of B0. With temperature fluctuation, oxygen consumption rates gradually increased and then were higher than B0 in the end. Ammonium excretion rates (AER) decreased with temperature fluctuation, except B1 of the small sized scallop. For the big sized scallop, the AER in B3 was significantly lower than that in B0 (P<0.05), where for medium sized scallop, AER in B4 was significantly lower than that in B0 (P<0.05). Comparing with physiological parameters, the immune activities were more sensitive to temperature fluctuation. Activities of superoxide dismutase (SOD) and catalase (CAT) decreased significantly as the temperature fluctuated 1 or 2 times (ANOVA, P<0.05). In conclusion, the temperature fluctuation in optimum range had impact on the physiological parameters and immune activities of scallop P. yessoensis to a certain extent.