Abstract: To investigate the effect of nitrate and phosphate on phytoplankton growth, we carried out field experiments and enrichment incubation experiments in the adjacent waters of the Xisha Yongxing Islands (S0 station: 17°30.364′N, 111°59.843′E) in late fall of 2014. The added nitrogen concentrations were 4 μmol · L^-1, 8 μmol · L^-1, 16 μmol · L^-1 and 32 μmol · L^-1; the added phosphorus concentrations were 0.25 μmol · L^-1, 0.5 μmol · L^-1, 1 μmol · L^-1 and 2 μmol · L^-1; the added mixed nitrogen and phosphorus concentrations were 4 μmol · L^-1+0.5 μmol · L^-1, 8 μmol · L^-1+2 μmol · L^-1, 16 μmol · L^-1+0.25 μmol · L^-1 and 32 μmol · L^-1+1 μmol · L^-1. The results show that the chlorophyll a increased significantly after adding mixed nitrate and phosphate (P < 0.05). The chlorophyll a concentration increased from 0.06 mg · m^-3 to 1.71 mg · m^-3 and 1.10 mg · m^-3 at S0 station after adding 32 μmol · L^-1NO₃^--N+1 μmol · L^-1PO₄^3--P and 4 μmol · L^-1NO₃^--N+0.5 μmol · L^-1 PO₄^3--P mixed nitrate and phosphate, respectively. It is suggested that enrichment of mixed nitrate and phosphate (N/P: 8~32) could promote the growth of phytoplankton. However, enrichment of single nitrate and single phosphate did not promote the phytoplankton growth because the chlorophyll a concentration showed no significant increase by adding single nitrate and single phosphate at S0 station. Consequently, the restriction factors of phytoplankton growth at S0 station were both nitrate and phosphate concentrations.