The effects of environmental factors on seasonal and annual variations in soil respiration were examined in the cool temperate Zoysia japonica grassland of Japan. Field measurements of soil respiration were conducted using a closed chamber method with an infrared gas analyzer at monthly intervals in the snow-free seasons from May 2007 to December 2009. There was an exponential relationship between soil respiration and soil temperature, and the soil temperature accounted for 85-86% of seasonal soil respiration variability. Moreover, a positive linear relationship between soil respiration and soil water content was detected in summer (R (2) = 0.55, p < 0.001), but not in spring or autumn. Annual soil respiration was estimated at 755, 719, and 1,037 g C m(-2) year(-1) in 2007, 2008, and 2009, respectively. These interannual variations in soil respiration might be influenced by the strength of precipitation during rainy seasons and the timing of each snow-melt. Our results suggest that the effects of rainfall and snow-melt events on soil respiration might be important factors to understand carbon dynamics in grassland ecosystem in Japan.