Experimental studies at the Lhasa agroecological station of the Chinese Academy of Sciences during growth seasons of 1995～1996 indicated that the CO2 emission flux of soil in the farmland and pasture land in Tibetan Plateau was 1 7～10 5kgCO2ha-2h-1(1 6～6 6μmol CO2m-2s-1). CO2 emission flux of soil under several vegetation covers during daytime is higher than that at night, and afternoon higher than the morning. During the growth period of crops and herbage, the CO2 emission amount in the farmland is about of that absorbed by plant photosynthesis. As raw materials for photosynthesis, the soil emitted CO2 forms organic matter for the plants, acting as carbon fixation. CO2 emission amount can not lead to CO2 concentration increase in the atmosphere. Factors affecting CO2 emission flux of soil are mainly the factors of growth period, vegetation type and environment. In terms of environmental factor, the CO2 emission flux of soil in the farmland is positively correlated with air and soil temperature (soil surface temperature at 0cm,land underground temperature at the depth of 5cm and 10cm), and is negatively correlated with air pressure and atmospheric CO2 concentration.