Decomposition processes of leaf litter were observed during one year with same kinds of substrates at a warm-temperate evergreen oak forest in Japan and a tropical rain forest in Sumatra simultaneously. A warm-temperate forest showed high rates of carbon-weight loss in summer. A tropical rain forest had high rates at the initial stages in particular and apparently low rates during the severe dry weather in early 1997. When time trends of carbon-weight loss could be approximated by a single exponential curve for each leaf litter, the average values of decomposition factor of five Japanese fagaceous leaf litter were 0.70±0.13 y^-1 at a warm-temperate forest and 1.62±0.24 y^-1 at a tropical rain forest, respectively. The C/N ratios of litter gradually decreased with the progress of decomposition at the initial stages, then tended to be constant at about C/N =20 at the last stages irrespective of kinds of leaf litter and study sites. Basing on these results we discussed characteristics of decomposition at these two forest ecosystems.