The effects of different nitrogen concentrations on the growth and some chemical compositions in a marine diatom,Phaeodactylum tricornutum,have been examined in laboratory cultures by employing low nitrogen(LN,44 μM NaNO3),medium nitrogen(MN,880 μM NaNO3)and high nitrogen(HN,4400 μM NaNO3)treatments.Our results indicated that growth curve,specific growth rate and cell biomass of Phaeodactylum tricornutum were significantly limited under LN condition.Although HN treatment significantly enhanced the algal cell number and cell yield in the stationary phase,it did not show remarkable differences in the growth curve and specific growth rate in the early stages as compared with MN treatment.Besides,the contents of soluble sugar,protein and chlorophyll a were obviously increased with the increasing of nitrogen concentrations.It can be concluded from the present investigation that higher nitrogen concentrations can accelerate the growth arnd division as well as biosynthesis of some chemical compositions ofPhaeodactylum tricormutum,and therefore this should be taken into account for the prevalence of red tides dominated by the marine diatom Phaeodactylum tricornutum in recent years.