Forest plays an important role in carbon cycle and global climate change. To estimate accurately net ecosystem carbon exchange (NEE) between forest ecosystem and the atmosphere and understand the ecophysical factors influencing NEE, it is necessary to study the variation characteristics of CO_2 storage flux. In this study, the eddy covariance method and the profile method were used to measure CO_2 flux and CO_2 concentration over a warm-temperate mixed plantation in the Xiaolangdi area in 2008,respectively. The temporal and spatial variations of CO_2 concentration under sunny and cloudy sky conditions above the forest canopy were analyzed. CO_2 storage flux obtained by the eddy covariance was compared with those derived from the profile method and the diurnal and seasonal variations of CO_2 storage flux were investigated. The results showed that the diurnal and seasonal variations in CO_2 concentration were obvious. In the sunny days of the growing season, CO_2 concentrations above the plantation canopy decreased continuously and the minimal CO_2 concentration appeared at 12:00. After 12:30,CO_2 concentration increased slowly. However, in the cloudy days, the maximal and minimal CO_2 concentrations occurred in the early morning and at about 14:30, respectively. At the annual scale, CO_2 concentration peaked in March, with a value of 370 mumol/mol. The minimal CO_2 concentration occurred in August, with a value of 347 mumol/mol. CO_2 storage flux estimated by the eddy covariance method was 9% lower than that obtained by the profile method. During the growing season of 2008,monthly mean CO_2 storage flux was -0.0004 mg CO_2 m~(-2) s~(-1) and NEE was -0. 091 mg CO_2 m~(-2) s~(-1). At the daily scale, the proportion of CO_2 storage flux to NEE was only 0.4%. In 2008, CO_2 storage flux and NEE were -46.1 and -1133 g CO_2 m~(-2) a~(-1), respectively. On an annual scale, CO_2 storage flux only accounted for 4.1% of NEE. Therefore, at the longer time (daily or annual) scale, CO_2 storage flux could be neglected when NEE is estimated in the mixed plantation of the Xiaolangdi area.