There is a general agreement that forest ecosystems in the Northern Hemisphere function as significant sinks for atmospheric CO2; however, their magnitude and distribution remain large uncertainties. In this paper, we report the carbon (C) stock and its change of vegetation, forest floor detritus, and mineral soil, annual net biomass increment and litterfall production, and respiration of vegetation and soils between 1992 to 1994, for three temperate forest ecosystems, birch (Betula platyphylla) forest, oak (Quercus liaotungensis) forest and pine (Pinus tabulaeformis) plantation in Mt. Dongling, Beijing, China. We then evaluate the C budgets of these forest ecosystems. Our results indicated that total C density (organic C per hectare) of these forests ranged from 250 to 300 t C ha-1, of which 35―54 t C ha-1 from vegetation biomass C and 209―244 t C ha-1 from soil organic C (1 m depth, including forest floor detritus). Biomass C of all three forests showed a net increase, with 1.33―3.55 t C ha-1 a-1 during the study period. Litterfall production, vegetation autotrophic respiration, and soil heterotrophic respiration were estimated at 1.63―2.34, 2.19―6.93, and 1.81―3.49 t C ha-1 a-1, respectively. Ecosystem gross primary production fluctuated between 5.39 and 12.82 t C ha-1 a-1, about half of which (46%―59%, 3.20―5.89 t C ha-1 a-1) was converted to net primary production. Our results suggested that pine forest fixed C of 4.08 t ha-1 a-1, whereas secondary forests (birch and oak forest) were nearly in balance in CO2 exchange between the atmosphere and ecosystems.