Abstract: We investigated ecosystem biomass in a chronosequence of Eucalptus urophylla × E. grandis plantation forests (2-, 3-, 4-,5- and 6-year-old) in the mountain region of southern Fujian Province. The biomass of trees was determined by average tree sampling method, that of understory vegetation by harvesting grass and shrubs, and that of forest floor by collecting litter from subplots. The results showed that tree biomass was estimated to be 45.14, 69.22, 92.20, 111.82 and 175.09 t/hm2, respectively, in the 2-, 3-, 4-, 5- and 6-year-old plantations, with stem wood making up the largest proportion (44.5%~60.4%) and fine roots the smallest (0.2%~0.65%). The net primary productivity was 22.86, 26.07, 32.63, 32.70 and 46.69 t/(hm2•year) in the 2-, 3-, 4-, 5- and 6-year-old stands, respectively, with leaf productivity increasing from 2- to 4-year-old stage but decreasing during the later growth phase. The understory biomass was 5 308.83, 7 201.62, 5 676.47, 4 033.42 and 9 183.13 kg/hm2 in the age sequences, among which shrub biomass reached the peak at the age of 3 (2 121.89 kg/hm2) and grass biomass topped at the age of 6 (1 062.19 kg/hm2). The standing crop of the forest floor decreased in the age order of 6 years 3 years 5 years 2 years 4 years, with the undecomposed litter occupying greater proportion than the partially decomposed in the early ages (2- to 5-year-old).