Abstract: Effect of different gradients of seawater salinity(0～35 g/L)and temperature(20～35℃)on seed germination of mangrove tree species, Sonneratia apetala Buch. Ham. was reported in the paper and the results showed that the discrepancy among them was remarkable. Generally benefitial conditions for seed germination were temperature higher than 25℃,salinity below 10.0 g/L. The best condition for seed germination was salinity gradient from 2.5 g/L to 5.0 g/L and temperature 30～35℃. When salinity was among 10～15 g/L,seed germination and young seedlings growth apparently declined;when salinity was higher than 15 g/L,only a few seeds could germinate and young seedlings grew far slowly and afterwards gradually died with salinity damage in culture dish but no seeds could germinate in mudflat field nursery. Spraying freshwater on mudflat field nursery could improve seed germinating percentage to a great degree. Under low seawater salinity such as 2.5～7.5 g/L,the content of Na+ and Cl- assimilated by young seedlings gradually increased with time and then reached dynamic balance in 20 days.