In order to investigate nutrient limitation of phytoplankton in natural marine ecosystems, in-situ nutrient enrichment experiments with nitrate, ammonium, phosphate and iron addition were conducted in Jiaozhou Bay in August November, 1999 and February May and August, 2000. Jiaozhou Bay, surrounded by Qingdao city and with area of 390 km2 and mean depth of 7m is an eutrophic coastal bay. The concentrations of nitrate, ammonium and phosphate were 1.98, 8.01and 1.38 μmom/L in the 1999s. DIN: P molar ratio was almost always over 16 (the Redfield ratio, also called “geochemically balanced”) in all seasons. Five nutrient enrichment experiments were conducted to investigate nutrient limitation on phytoplankton. Size-fractionated chlorophyll-a results showed that netphytoplankton (20-200μmom/L) showed most significant response to N addition; nanophytoplankton (2-200μmom/L) showed significant response to P addition, but N addition also stimulated their growth; picophytoplankton (<2μmom/L) showed positive response to N or P addition, but this response was not more significant than that of netplankton or nanoplankton. N probably Limited phytoplankton growth in summer, but P and N nutrient could phytoplankton growth in other seasons in Jiaozhou Bay.