Abstract: Based on the 7 times of ZA section surveys in Dong Sea from March 31 to May 27 in 2011, characterization of urea sectional distribution in this area and its affecting factors during the outbreak of Harmful Algal Blooms were analyzed. The results showed that the concentration of urea ranged from 0.06 to 3.27 mol/L during outbreak of diatom bloom from March 31 to April 28, mainly influenced by Changjiang dilution water. It decreased a lot and remained 0.12~2.71 mol/L when dinoflagellate red tide broke out in May, affected by metabolism of phytoplankton significantly. In the cruises, the high-value region of urea concentration presented characteristic of transferring far away from coastal area and then slowly closing the inshore. The whole urea contents showed a decreasing trend with the passing of time. The level of urea concentration during outbreak of dinoflagellate red tide was less than that when diatom red tide outbroke and its low area was corresponded with the high values of the concentration of chlorophyll, suggesting that the effective absorption and utilization of urea for dinoflagellate was one of the reasons to maintain a high incidence of dinoflagellate red tide in East China Sea.