Abstract: Sawtooth oak( Quercus acutissima )is one of the most important afforestation tree species.To study its photosynthetic characteristics and its response to elevated CO 2 can provide the foundation for predicting the production of sawtooth oak forest,making its carbon budget and understanding its response to global changes.The experiment was carried out with Licor 6400 portable photosynthesis system.The main results were as follows:(1)The critical points of light compensation and saturation of sawtooth oak were about 50 μmol/(m 2·s) and 1 200 μmol/(m 2·s) respectively,which meant that sawtooth oak was a light demanding plant.Its CO 2 compensation point and saturation point were 60 μmol/mol and 1 200 μmol/mol respectively,which meant that it was of a C 3 type plant.(2)The daily course of net photosynthesis rate( Pn )was single peaked during spring and autumn,however it was double peaked during summer.The daily maximum Pn in spring,summer and autumn was 7.52, 14.03 and 7.43 μmol/(m 2·s) respectively.(3)Principal component analysis showed that the leading factors affecting Pn during spring were photosynthetic active radiation( PAR )and sample CO 2 concentration( Cs ),which during summer were vapour pressure deficit( VPD )and leaf temperature,which during autumn were VPD and humidity.(4)Compared to the average Pn under normal CO 2 concentration,which was 7.55 μmol/(m 2·s),the average Pn of sawtooth oak under doubled CO 2 concentration increased 89.17%,which was 17.03 μmol/(m 2·s).