Diurnal variation in CO2 flux from the soil was measured by a closed chamber technique at a developed land site in Tokyo from September to November in 1999. The measurements were made four times for 24-h periods at two fixed sites where there was no plant cover (A) and where Miscanthus sinensis was dominant (B). CO2 fluxes averaged 98.2mg CO2 m-2h-1 at the site A and 184.3mg CO2 m-2h-1 at the site B. The fluxes from both sites showed a clear diurnal variation, which was high during the day and low at night. The temporal variation was highly correlated with the air and soil temperatures, especially at depths of 1 and 5cm. The relationship between CO2 flux and temperature was exponential. The CO2 flux increased exponentially 1.99 times when the soil temperature at 1cm deep increased by 10°C.