Diatoms from the surface sediments of the Northwestern South China Sea were studied, aiming at revelation of the relationship between the distribution pattern of diatom and the geo-environment, and the usage of diatom for interpretation of the marine environment. The results indicate:a) the diatom floras are dominated by tropical-subtropical species and coastal-neritic species in this case. b) The abundances of diatoms are quite different regionally. They are lower in the shallow water of offshore area in general, but higher in the slope, estuary and upwelling regions. c) Diatom's distribution patterns are obviously controlled by environment factors, such as temperature, salinity, nutrient, water depth, current and substrate, and so on. d)Diatom assemblages are complicated and could be used to indicate the current system and water mass movement.