Abstract: The paper deals with the biomass of average 20-aged Bruguiera gymnorrhiza stand. The vertical stratification method was adopted to measure the above ground biomass of different size-class trees in the stand. The tree mean growing area method was adopted for measuring the root biomass. The total biomass and the biomass of different tree organs within the stand were obtained by using both allometric method and the mean tree method. Besides, the leaf area of the stand was measured and the leaf area index was calculated so as to evaluate the productivity of the stand. In order to provide a theoretical basis for mangroves management and utilization, the yearly dynamic variation of the mean and annual net production and the variation tendency of the net assimilation rate with the increasing of the stand age have been analysed.