The relationships between reproductive allocation and different factors have been widely recognized for terrestrial plants, but scarcely for marine angiosperms. Focusing on the seagrass Zostera marina, we examined the change in biomass when it was partitioned to growth, vegetative propagation and reproductive allocation of Z. marina. We also evaluated the relationships between reproductive allocation and biotic and abiotic conditions in the coastal areas of north China. The results indicated that the reproductive allocation of Z. marina at the levels of genet, shoot and spathe was positively correlated with shoot characteristics such as height and biomass ( P < 0.01), but was negatively correlated with branch number and leaf number per genet ( P < 0.01). In addition, the allocation was also affected by other abiotic factors such as water column phosphate concentration ( P < 0.05) and sediment total petroleum ( P < 0.01). These findings indicated that both biotic and abiotic conditions significantly influenced the reproductive allocation of Z. marina, which may affect further the population dynamics.