Abstract: In this study, effects of three different nitrogen sources on the growth and interspecific competition of Phaeodactylum tricornutum, Dunaliella salina and Karenia mikimotoi were studied in the monoculture and mixed culture, respectively. The results indicated that when the three microalgae were in monoculture, the maximum final cell density of P. tricornutum was recorded in urea treatment, in which the average specific growth rate of P. tricornutum that reached 0.63 /d was the highest in all treatments. The final cell densities of D. salina and K. mikimotoi in NaNO3 and urea treatments were both higher than those observed in NH4Cl treatment, but there was no significant difference between the former two treatments, and the higher values of the average specific growth rates of D. salina and K. mikimotoi were also detected in the former two treatments. When the three microalgae were in mixed culture, the higher final cell densities of P. tricornutum were detected in NaNO3 and urea treatments, accounting for 62% and 65% of the total cell densities, respectively. And the average specific growth rates of P. tricornutum in NaNO3 and urea treatments were significantly higher than that observed in NH4Cl treatment. The maximum final cell density of D. salina that accounted for 58% of total cell density was recorded in NH4Cl treatment, in which the average specific growth rate was significantly higher than those observed in other two treatments. The growth of K. mikimotoi under three different nitrogen sources were all inhibited to a great extent, the final cell density which only accounted for 1% of the total cell density and the average specific growth rate that reached 0.058 /d in NH4Cl treatment were both the lowest in all treatments.