Two dominant microalgae species in the East China Sea (ESC), Chaetoceros curvisetus(C. curvisetus) and Karenia mikimotoi (K. mikimotoi), were chosen to study the effects of temperature on microalga growth, nitrate reductase activity (NRA), and their relationship under laboratory set-up. At the beginning of the incubation, DIN, PO_{4-P, and SiO3-Si were added to 4 500 ml sea water in 5 L bottles to form a final concentration of 32 μmol/L, 1.5 μmol/L and 32 μmol/L, respectively. The bottles were then incubated under ambient sunlight(about 50 W·m^-2) at different temperatures (10℃, 15℃, 20℃, 25℃, and 30℃) for 10～20 days. Triplicates were employed for all treatments. The microalgae cell numbers were counted by hemacytometer counting method. S-logistic 2 population growth model was used to simulate microalgae growth , and calculate two growth-related parameters, the maximum growth rate (μmax) and the final biomass (Bf). The activity of nitrate reductase was measured in vitro and its relations to microalgal growth were elucidated. The two microalgae were observed to adapt to different temperature ranges. C. curvisetus could grow normally in the range of 10℃～30℃, while K. mikimotoionly grew well between 15℃～25℃. Both μmax and Bf reached the peak values at 20℃ for C. curvisetus and at 25℃ for K. mikimotoi, respectively. These results indicated that compared to C. curvisetus, K. mikimotoi had a relatively narrow range of suitable temperature and a high optimum temperature (Topt). Throughout the incubation time, NRA of the two species changed similarly under different temperatures. NRA raised slowly at the first 3～5 days, and then dropped rapidly. The maximum NRA values (NRAmax) usually occurred at the early exponential growth phase, prior to appearing time of μmax and Bf. The relations of μmax, NRAmax and Bf with temperature followed the Shelford tolerance law. All above results suggested that temperature could affect the nitrate assimilation ability of microalgae, and subsequently affect the growth of algae. μmax and the NRAmax per unit volume of the C. curvisetus were higher than those of K. mikimotoi under the same environmental conditions, indicating that C. curvisetus might have a better ability of absorbing and utilizing nitrate for self-growth than K. mikimotoi.}