Abstract: The biomass of six-year-old Sonneratia apetala Buch-Ham. and Bruguiera gymnorrhiza (L.) Lamk. pure plantation in Nansha were surveyed with both the correlative growth method and the mean sample tree method. The result showed that the biomass of S. apetala was 1 146.08 t·hm-2, in which the aerial part was 869.17 t·hm-2 and the underground part was 276.91 t·hm-2; the biomass of the trunk was 473.79 t·hm-2, which was accumulated as the highest biomass and took up 41.3% of the total. The biomass distribution among organs was in the order of trunk>branch>root>bark>flower and fruit>leaf. The biomass of B. gymnorrhiza was 55.51 t·hm-2, in which 40.62 t·hm-2 for the aerial part and 14.89 t·hm-2 for the underground part, and the biomass distribution among organs was in the order of trunk>root>branch>leaf. The average net productivity of S. apetala was 191.02 t·hm-2·a-1 and the average net productivity of B. gymnorrhiza was 9.26 t·hm-2·a-1. As the population age and initial density of plantation are taken into consideration, both the biomass and productivity of six-year old S. apetala were higher.