Boreal forest is the second largest biome on the earth, which lies in middle-high latitude of the northern hemisphere and where temperature increases strongly. Thus, boreal forest is an important research object in global climate change and carbon cycle. Larix gmelinii forest in Daxing′an Mountains of Northeast China is an important component of boreal forest. With open-path eddy covariance and meteorological gradient observation system, net ecosystem CO2 exchange (NEE) and environmental factors of virgin Larix gmelinii forest in Daxing′an Mountains (123°01′04″ E, 51°46′52″N, 773 m a.s.l.) were observed from June to October in the years from 2006 to 2008. The results showed that the diurnal patterns of NEE and photosynthetically active radiation (PAR) could be both expressed as one-humped curves. The maximum of NEE appeared at 9:00-10:00 (Local time) while PAR was at about 11:30. The diurnal dynamics of NEE was not synchronized with diurnal pattern of PAR, and NEE could be inhibited under high light intensity. Virgin Larix gmelinii forest ecosystem was a carbon sink in June, July and August, while a carbon source appeared in September and October. The average NEE from June to October was -0.082, -0.082, -0.061, 0.009, and 0.014 mgCO2 ?m-2 ?s-1, respectively. The daily length of carbon sequestration from the atmosphere was reduced from 14 hours (5:00-19:00) in June to 9 hours (7:30-16:30) in October. The parameters of light response curves of NEE from June to October in the years from 2006 to 2008 were fitted with Michaelis-Menten equation. The maximums of apparent quantum yield (α), maximum ecosystem assimilation rate (Pmax) and ecosystem respiration rate (Rd) were 0.0032 mgCO2 ?μmol photon-1, 08554 mgCO2 ?m-2 ?s-1 and 0.2734 mgCO2 ?m-2 ?s-1 in virgin Larix gmelinii forest, which were smaller than those in Changbaishan temperate mixed forest. It indicated that photosynthesis and respiration in temperate mixed forest ecosystem were more active than those in cool temperate Larix gmelinii forest ecosystem. The fitted ecosystem respiration rate (Rd) ranged from 0.03 mgCO2 ?m-2 ?s-1 to 0.27 mgCO2 ?m-2 ?s-1 from June to October, being similar to the observed average nighttime ecosystem respiration rates ranging from 0.07 mgCO2 ?m-2 ?s-1 to 0.24 mgCO2 ?m-2 ?s-1. The light response curves of NEE in the four different temperature regimes showed that the optimum temperature was from 20 ℃ to 30 ℃, and the maximum of NEE was -0.43 mgCO2 ?m-2 ?s-1. The results would provide a new insight into the carbon cycle of terrestrial ecosystem and its controlling mechanism.