Nitric oxide was discovered in both the lab and the alga culture pond of Daya Bay (1-300 m3) before the growth of alga reached the maximum. The results included: (1) NO was detectd before the growth of alga reached the maximum in the case of red tide alga and food alga, and the concentration of NO decreased rapidly after the growth maximum; (2) the curve between NO concentration and time indicated that the concentration of NO in the daytime was more than that at night,and the maximal concentration of NO appeared in the midday (1-3 pm); (3) the growth of alga reached the maximum in the alga culture pond of Daya Bay in about 8- 10 d, and NO was discovered in 5-7 d; (4) the measured NO concentration was 10-9 mol/L, 10-9-10-8 mol/L, and 10-8 mol/L for Haeterosigma akashiwo, mixed alga in Daya Bay and Chaetoceros Curvisetus individually; (5) the relation of illumination with NO production was discussed.