The CO2 flux of a wet sedge tundra ecosystem in the Arctic, at Barrow, Alaska, has been measured by the eddy correlation method since spring 1999, and the CO2 uptake by the vegetation during the spring and growing periods was examined between 1999 and 2000. CO2 flux changed to a sink immediately after the spring thaw in 1999 and the photosynthetic activity was high in the first half of the growing period. At this time the air temperature was low and solar radiation was high. In the 2000 season, the temperature was approximately 5 oC lower during the snow-covered period, and increased up to 5 oC higher right after the spring thaw but the solar radiation decreased to two thirds of that in 1999. Thus, we found different CO2 accumulation during the snowmelt and the following two weeks between both years. The difference in the climate at beginning shoulder period of the growing season resulted in the difference of CO2 accumulation through the growing period. The maximum level of photosynthetic potential (Pmax) in late July was analyzed as being almost the same at 20 gCO2 m−2 d−1 for both years. However, the weekly average peak CO2 uptake was 16.4 and 11.9 gCO2 m−2 d−1 in 1999 and 2000, respectively, with the lower number in 2000 caused by the low radiation with high air temperatures. The CO2 accumulation during the spring and through the growing periods was a net sink of 593 gCO2 m−2 in 1999 and a sink of 384 gCO2 m−2 in 2000. High CO2 accumulation in 1999 was caused by earlier development of the vegetation, and the lower CO2 uptake in mid summer in 2000 was caused by unseasonable weather.