Based on the statistical consideration for air-sea sensible and latent heat and momentum fluxes estimated from the data obtained at OWS-T (29°N, 135°E) from June 1950 to November 1953, the relationship between the standard error of monthly mean fluxes and the number of the data used in estimation was examined. The standard errors varying seasonally for the number of the data were presented, which were given by the statistical theory, i.e., the standard deviation of the population divided by the square root of the number of the sampled data. These results were also confirmed by the computer simulation for randomly sampled and small-in-number data, and they holded even though the distribtuion of the population deviates from the normal one. From the actual number of the marine meteorological data compiled by the Japan Meteorological Agency from 1961 to 1985, it could be expected that in the sea south of Japan, momentum fluxes can be estimated with accuracies of 0.15 and 0.1 dyn⋅cm^-2 in winter and summer, respectively, the sensible heat fluxes 5 and 1 W⋅m^-2 and the latent heat fluxes 15 and 5 W⋅m^-2.