Abstract: The effects of different P concentrations(0, 10, 36.3, 290.4 μM)on the chlorophyll fluorescence parameters, cell density and relative chlorophyll content of Chlorella sp. were reported.The algae were obtained from the Microalgae Culture Center (MACC), Ocean University of China.The cultures were grown at 20±1℃, the salinity was adjusted to 31 by using distilled water, and the light intensity was 100 μmol · m^-2 · s^-1.One-way analysis of variance showed that P concentrations have significant effects on the photosynthesis and growth of Chlorella sp.with the culture age increasing(P < 0.05). For Chlorella sp., F_v/F_m, ΦPSII, ETR, qP, cell densities and chlorophyll contents all increased with the increase of P concentrations and reached their highest values at 36.3 μM.Multiple comparison tests showed that the optimal P concentrations for the photosynthesis and growth of Chlorella sp. is 36.3 μM.The chlorophyll fluorescence parameters showed characteristic NIFT responses following PO₄^3- addition to P-starved Chlorella sp., fluorescence decreased but qP, NPQ increased steadily, ΦPSII increased sharply, and then a decrease to the same steady state as that prior to addition.Seven days later, the lacked nutrient P was added and chlorophyll fluorescence parameters were measured every 2 hours.The results showed that F_v/F_m, ΦPSII, ETR of Chlorella sp.were all recovered in 24 hours.The changes of qN, NPQ were not significant.