Nitrogen is the essential element for phytoplankton. Its concentration and form affect the biomass and community composition of phytoplankton directly or indirectly. In order to discover the restriction of different forms of nitrogen on phytoplankton growth in Lake Taihu, and supply scientific ground for further study of cyanobacteria blooms and eutrophication control, the especial uptake rates of ammonium, nitrate and urea by phytoplankton using ¹⁵N-tracer technology as well as physicochemical parameters, urea concentration and phytoplankton community in Mailiang Bay were determined in the middle of January, April, July and October. The result showed that the uptake of ammonium, nitrate and urea, except the uptake of ammonium in August, exhibited the classic hyperbolic Michaelis-Menten kinetic curves. The maximum uptake speed rates (V_max) of three forms of nitrogen were:ammonium > nitrate > urea in winter and spring while ammonium > urea > nitrate in summer. The differences of V_max between seasons and forms were owing to the different phytoplankton community compositions and ammonium concentration in water. According to the K_s, the growth of phytoplankton community was more likely to be limited by nitrate and not prone to be limited by urea in winter and spring, while more likely to be limited by ammonium in summer. In autumn, it was prone to be limited by urea other than nitrate. In a word, the growth of phytoplankton community was limited by different forms of nitrogen in different seasons.