Abstract: Gas exchanges of eight poplar clones were compared during the growing season in l992.Clonal and seasonal variations in leaf net photosynthetic rate(Pn),stomatal conductance,transpiration as well as the environmental factors(quantum,temperature,relative humidity etc.)were measured.There were significant difference(0.05 and 0.01 level)of Pn among clones from July to September,besides,through Duncan's multiple range test and correlation analysis,they were divided into three groups:the higher group of Pn was Populus×xiaozhuanica cv.‘Zhaolin-6,P.×xiaozhuanica cv.‘poplaris’,P.×xiaozhuanica cv.‘Chifengensis-34’,and P.×euramericana cv.‘Agethe F’(from 14.757 to 14.055μmol CO2 m-1s);the middle one was P.×xiaohei(13.264 μmol CO2 m-2s-1),and the lower one was P.×pseudo-simonii,P.×deltoides cv.‘Imperial’ and P.×euramericana.‘N3061’(from 12.369 to 11.773 μmol CO2m-2s-1).Among them,clones with the highest Pn,especially P.×xiaozhuanica‘Zhaolin-6’was highly effected by the light rather than the other ervironmental factors,therefore it was suitable for the cultivation in local areas.However,the others with the lowest Pn were limited by humility,vapor pressure,and even light.Therefore,the variations of Pn among clones from different sites and its limited factors were identified in the semi-arid areas.