A subtropical mangrove along the Miyara River in Ishigaki Island, Japan was studied for evaluating the carrying capacity for biomass of the monospecific stands. Rhizophora stylosa and Bruguiera gymnorrhiza were dominant in the downstream area whereas B. gymnorrhiza in the upstream. The stem diameter D, stem height H, fine roots mass were measured and, aboveground biomass AGB, belowground coarse root biomass BGBcoarse were estimated. The AGB, BGBcoarse and fine root mass were estimated as 128.46 Mg ha-1, 31.01 Mg ha-1 and 12.75 Mg ha-1 in the R. stylosa; 269.82 Mg ha-1, 93.68 Mg ha-1 and 11.13 Mg ha-1 in the downstream B. gymnorrhiza; and, 227.94 Mg ha-1, 81.05 Mg ha-1 and 6.35 Mg ha-1 in the upstream B. gymnorrhiza plots, respectively. The AGB did not differ among the plots, meanwhile BGBcoarse was significantly lower and fine root mass was significantly higher in the R. stylosa plots than in the downstream B. gymnorrhiza plots. Significantly lower mean individual phytomass wt specific to tree density  of R. stylosa plots than B. gymnorrhiza plots in the  – wt relationship was found, which denoted the lower carrying capacity for AGB of R. stylosa than that of B. gymnorrhiza. The results rejected our hypothesis that the stressful edaphic conditions, such as high soil salinity and low pH at the downstream, limit biomass and potential canopy height Hmax of mangrove along a river gradient but partly supported another hypothesis that biomass and Hmax differs between different mangrove species at the same edaphic environment.