A Skeletonema costatum red tide occurred on July 3—8, 1998 in Jiaozhou Bay. Variations of ecological factors and their influence upon red tide organisms were determined. Relatively high contents of nutrients, iron and manganese were responsible for the growth of Skeletonema costatum. The outbreak of the Skeletonema costatum red tide has a close relationship with the strong rain on June 30 and July 1, 1998. After the rain, warm, low-salinity and eutrophic water resulted in the rapid proliferation of Skeletonema costatum. In terms of the nutrients structure, compared with TIN (total inorganic nitrogen), the concentration of silicate and phosphate were relatively deficient. At the developing stage, the concentration of various nutrients and iron in surface water as well as TIN were declining due to the declination of nitrate. Among the three forms of TIN, it is possible that Skeletonema costatum preferred to absorb nitrate. At the blooming stage, the exhaustion of phosphate and silicate led to the death of Skeletonema costatum and disappearance of red tide. After the bloom disappeared, the contents of TIN and silicate returned rapidly to normal, but the concentration of phosphate still remained low-level and renormalized after a considerable time. Two statistical methods (correlation analysis and principal component analysis) were used to determine the mechanism of red tide. Combined results show that temperature, salinity, silicate and phosphate were the critical ecological factors affecting the occurrence of the Skeletonema costatum red tide.