Abstract: In order to analyze the differences on photosynthetic characteristics of tetraploid and diploid Lagerstroemia indica L.and develop high- quality forest, CIRAS-1 portable photosynthesis system was employed to measure various photosynthetic physiological parameters and their variation regularities in tetraploid and diploid Lagerstroemia indica L.The results showed that: Both the diurnal change of net photosynthetic rate ( Pn ) of etraploid and diploid Lagerstroemia indica L. showed a single- peak curve,but the peak value appeared at different hours. The net photosynthetic rate, stomatal conductance and water use efficiency had significant difference between the tetraploid and diploid Lagerstroemia indica L.and the transpiration rate, apparent quantum yield and carboxylation efficiency had no significant difference . Light saturation point ( LSP) of diploid Lagerstroemia indica L.[1445μmol/(m2?s)]was lower than that of tetraploid Lagerstroemia indica L.[1670μmol/(m2?s)] and light compensation point ( LCP) of diploid Lagerstroemia indica L.[12.23μmol/(m2?s)] was lower than that of tetraploid Lagerstroemia indica L.[42.2μmol/(m2?s)].CO2saturation point (CSP) of diploid Lagerstroemia indica L.[1200μmol/mol] was lower than that of tetraploid Lagerstroemia indica L.[1427.5μmol/mol] and CO2saturation point ( CCP) of diploid Lagerstroemia indica L.[32.24μmol/mol]was lower than that of tetraploid Lagerstroemia indica L.[35.71μmol/mol]．The chlorophyll content of unit leaf area of tetraploid Lagerstroemia indica L. were significantly higher than that of the diploid Lagerstroemia indica L. ( P ＜0. 05) ．