With rich nutrients, suitable temperature, light, and weather conditions, large-scale harmful algal blooms (HABs) have frequently occurred in the East China Sea in recent years. According to datas from the State Bureau of Oceanic Administration of China from 2009 to 2014, there were 367 instances of HABs in the coastal waters of China and 195 instances in the East China Sea, accounting for 53%. It is generally believed that the occurrence of algal blooms is linked to eutrophication of coastal waters. Nutrients are the basic material for the growth and reproduction of organisms. As an important kind of DON(Dissolved Organic Nitrogen), urea has high biological activity. Many studies have reported that urea could serve as the sole nitrogen source for a variety of phytoplankton. As such, studying the effect of urea on the organisms that induce red tides is very important. To determine the different responses of diatoms and dinoflagelattes to DON, the effect of urea on the growth of Skeletonema costatum and Karenia mikimotoi, which are the dominant algae in China, were determined using a batch of culture experiments in the laboratory. Results indicated that the two algae could grow under different ratios of urea to nitrate, but their kinetic parameters were different. With the increase of the proportion of urea in the mixed nitrogen source in the culture solution, the growth rate(0.91-0.82/d) of Skeletonema costatum and its maximum biomass(2.0×10⁵-1.2×10⁵ cells/mL) in the plateau phase of the growth curve gradually decreased. However, the growth rate (0.36-0.51/d) of Karenia mikimotoi gradually increased and its maximum biomass (1.1×10⁴cells/mL) in the plateau phase remained relatively unchanged. In plateau phase, the concentration of nitrogen in the Skeletonema costatum culture solution declined to 2.5μmol/L, and in Karenia mikimotoi it declined to 1.0 μmol/L. In the exponential phase, the concentrations of DON increased with the growth of algae, and reached the maximum values (5-6 μmol/L), then remained constant in the cultural of Skeletonema costatum. For Karenia mikimotoi, the concentrations of DON declined after reaching maximum values (2-3 μmol/L) in the culture solution. The content of particulate organic nitrogen in each cell of Skeletonema costatum (approximately 10^-6 μmol in the exponential phase, 10^-7 μmol in the plateau phase) were much less than that of Karenia mikimotoi (10^-4 μmol in the exponential phase, 10^-6 μmol in the plateau phase). In an environment with lower inorganic nitrogen and higher DON concentration, dinoflagellates exhibit better adaptability than diatoms. This research can provide references on the interpretation of the succession between diatoms and dinoflagelattes in the Yangtze River Estuary in spring.