Abstract: The quantity and species of decomposers including the microorganisms and animals in the soil of Casuarina eguisetifolia plantation ecosystem in two different ages on Dongshan island of Fujian Province were studied. The results showed that there were significant differences in the quantity of soil microorganisms between the plantations with different ages. Actinomycetes accounted for 50.65% of the total in the 16 years old plantation, which was the largest percentage, Bacteri accounted for 64.26% in the 25 years old plantation, which was the largest percentage. The quantity of bacteria and fungi showed growing tendency with the increase of plantation age, whereas actinomycetes changed with a contrary tendency. Ammonifier accounted for the largest proportion among the functional bacteria in both of the two different aged Casuarina eguisetifolia plantation ecosystem, which was 9161% in 16 years old plantation, and 86.42% in 25 years old plantation. The number of Azotobacteria increased sharply with the increase of age, but the number of organic phosphors decomposing microbes, nitrogen fixing bacteria and cellulose decomposing bacteria tapered off. There were 22 species belonging to 13 families and 7 orders of soil animals in the surface layer in the Casuarina eguisetifolia plantation ecosystem with two different ages.