As a part of ChinaFLUX projects, CO2 fluxes were measured below- and above-canopy of a temperate mixed forest at Changbai Mountain with the eddy covariance technique since late 2002. Using the CO2 fluxes data measured below- and above-canopy in 2003, this study quantified the contributions of below-canopy CO2 fluxes to ecosystem carbon budget of this mixed forest. The results showed that nighttime below-canopy CO2 flux increased exponentially with soil temperature at a depth of 5 cm, and below-canopy respiration was well consistent with soil respiration derived from chamber measurements (R2=0.77). The below-canopy respiration and soil respiration, coupling with gross ecosystem productivity (GEP) of the whole forest over the whole year, showed similar one-peak seasonal pattern over the whole year and reached their maximums in July through August. The annual below-canopy respiration and soil respiration were 770 g Cm-2a-1 and 703 g Cm-2a-1, respectively, contributing 59.88% and 54.69% to total ecosystem respiration. GEP of the below-canopy (GEPbc) showed clearly bimodal seasonal pattern, with higher rates in mid May and late August. Although GEPbc only accounted for 5.69% of the total forest photosynthetic productivity over the whole year, its contributions in seasons with low canopy density (i.e. April, May and October) reached 19.99%, 21.06% and 14.53%, respectively. The seasonal pattern of below-canopy net ecosystem productivity was controlled by the seasonal variability in respiration of this layer. The below-canopy acted as a net carbon source during the whole year, with peak carbon emission in August.