Abstract: The photosynthetic characteristics and water use efficiency of 17 Castanopsis hystix provenances were studied by measuring the gas exchange and δ13C values of the plant leaves. The results showed that the δ13C values of the 17 provenances varied in a range from -32.637‰ to -28.290‰, among which the δ13C values of the provenances from Donglan County, Pingxiang County and Rongxian County of Guangxi Zhuang Autonomous Region, and the provenance from Pu’er of Yunnan Province were relatively high, indicating that the water use efficiency of these provenances measured under the specific experimental conditions was high. The seasonlong water use efficiency (WUEsl) of all the provenances was lower than the corresponding instant water use efficiency. There were significant or extremely significant difference in the net photosynthesis rate (Pn) and WUEsl among the C. hystix provenances, but the differences in the intercellular CO2 concentration, stomatal conductance, transpiration rate indexes among the provenances did not reach significant level. Both photosynthesis rate (Pn) and intercellular CO2 concentration were significantly or extremely significantly and positively correlated with stomatal conductance, whereas extremely significantly and negatively correlated with transpiration rate. The seasonlong water use efficiency (WUEsl) of all the provenances was extremely significantly and negatively correlated with transpiration rate. The comprehensive analyses showed that the provenances from Donglan County and Rongxian County in Guangxi, Pu’er in Yunnan, Luhe County and Xinyi County in Guangdong Province were more drought resistant with higher water use efficiency.