The ecological amplitude of nitrate of algae can be defined in terms of the range of nitrate conditions under which it is able to grow and reproduce. It usually consists of the optimum nitrate concentrations, the optimum growth range of nitrate concentrations and the tolerance range of nitrate concentrations. In order to calculate the ecological amplitude of nitrate during algal growth, the relationship between nitrate concentration and the cell density, and that between nitrate concentration and maximum specific growth rate of Alexandrium tamarense were analyzed at three different fixed initial phosphate concentrations in laboratory cultures. The experiments were carried out in three independent batch cultures respectively. The cell density and the specific growth rate were assessed as the response parameters. A nitrate tolerance model of algal growth was established following Shelford's law of tolerance and using the experimental data. The results show that:(1) in low-P, medium-P, and high-P-grown cultures, both the cell density and the specific growth rate of A. tamarense increased when C_N ≤ C_Nopt and then decreased when C_N ≥ C_Nopt; (2) the nitrate tolerance model was consistent with Shelford's law of tolerance; (3) the optimum nitrate concentrations in low-P, medium-P, and high-P grown cultures were 30.36, 62.07 and 77.85 μmol/L, respectively; the optimum growth ranges of nitrate concentrations were 18.30-42.42, 37.71-86.43 and 41.52-114.18 μmol/L, respectively, and the tolerance ranges of nitrate concentrations were 6.24-54.48, 13.35-110.79 and 5.19-150.51 μmol/L, respectively. The results suggest that the ecological amplitude of nitrate during algal growth depended on the initial phosphate concentrations.