In situ incubation experiments of nutrients addition to surface water from the upwelling region in the Taiwan Strait were conducted to investigate nutrient limitation of this area and the main factors that caused the phytoplankton bloom in June, 2006. Variation of nutrients, Chl-a concentration and phytoplankton species density were analyzed. The results showed no clear limitation by silicon, however clear nitrogen and phosphate limitations were found, and nitrogen limitation was more serious than phosphate limitation. Diatoms, such as Asterionellopsis glacialis grew immediately to become the dominant species after nutrients were added. Different mechanisms were used to explain the assimilation of nitrogen and phosphate by these dominant diatoms. Nitrogen was taken up and assimillated immediately after it was added. Ammonium addition resulted in rapid growth and die-out of the dominant diatoms compared with that of nitrate addition. However, because of the phosphate pool inside phytoplankton, the added phosphate was taken up quickly and stored, then assimilated slowly. The results suggest that nitrogen supply was the main factor that caused the phytoplankton bloom in the Taiwan Strait during the upwelling in 2006.