Field cruises were carried out in the coastal waters of Hainan Island to survey urea concentrations and urease activities together with other environmental parameters. The bioavailability of urea for phytoplankton growth was analyzed based on these results. Results showed that the average concentrations of urea varied from 2.07 to 3.30 μmol/L, which accounted for 14%-38% of total dissolved nitrogen (TDN) and showed the trend to increase from the north to the south and the east. The urease activities of plankton were between 0.30-0.84 μmol N·L^-1·h^-1 with the maximum in the Haikou Bay. Most of the high values of urea concentrations and urease activities were detected in the nearshore waters where sewage draining exit, aquaculture area and the tourist area located. Diatoms were the dominant species in the phytoplankton communities. However, in some stations with dinoflagellates densities as high as 10⁴-10⁵ cells/L, the urea concentrations and the urease activities were also higher comparing with other sites. The phytoplankton biomass correlated well with the urease activities or urea/TDN atomic ratios which suggested the significance of urea as one nitrogen source in the coastal waters of Hainan Island. Urea and NH₄⁺ concentrations were the most important factors responsible for the distribution of dinoflagellates in the northern and the southern waters respectively. The terrestrial input of urea promoted the growth of phytoplankton, in particularly dinoflagellates, therefore urea might play an role in the competition and succession in the phytoplankton community in the coastal waters of Hainan Island during the spring period.