In order to clarify the community structures of litter-dwelling macrofauna and their effects on litter decomposition in subtropical forests of Irimote Island, I examined the following: (1) structures of macrofauna communities and seasonal variations in their abundance and biomass, (2) in situ decomposition rates (K) of organic matter by using litterfall-standing crop quotients, and (3) the effects of macrofauna on the decomposition rates of leaf litter by using litter bags of 2 mesh sizes. The study sites were a primary forest (Aira I) and 2 secondary forests (Station and Aira II). A total of 17,577 individuals were collected, and Formicidae was found to be the most abundant followed by adult Coleoptera and Isopoda. The biomass values were the largest in the case of Isopoda followed by Oligochaeta and Gastropoda. Most of the dominant animals did not show any clear trends in terms of seasonal variations in abundance and biomass, except for the biomass of Lepidoptera larvae that increased in the winter at all the 3 sites. There was little difference in total abundance of Aira I and Station, and these forests were approximately twice as abundance as Aira II. The biomass value was the largest in the case of Aira I; it was twice as much as the value in the case of Station and Aira II. The order of litterfall mass was as follows: Station>Aira I>Aira II. Further, the order of standing crop of organic matter on the ground was as follows: Aira II>Station>Aira I, and the mass of organic matter in the case of Aira I was ca. twice three as much as those in the case of Station and Aira II. The order of the decomposition rate (K) (Aira I>Station>Aira II) was similar to that in the case of the biomass of litter-dwelling macrofauna (Aira I>Station≥Aira II). Further, the litter bag experimental results showed that the decomposition rates of leaf litter were significantly higher in the case of coarse-mesh bags than in the case of fine-mesh bags at all the sites, and the leaf litter decomposition processes in the case of each mesh size was significantly different between the 3 sites. Furthermore, the contribution of macrofauna to the decomposition processes showed a different trend at each site. These results suggested that litter-dwelling macrofauna could affect the decomposition of organic matter on the ground in forests of Iriomote Islands.