Abstract: Using Li-6400 portable photosynthesis system, the photosynthesis, transpiration and water use efficiency (WUE) of Plarakmeria lotongensis (Chun et C. Tsoong) Law) were explored in this paper. The results showed as follows: (1) light compensation point (LCP) of Plarakmeria lotongensis (Chun et C. Tsoong) Law) was 65.5μmol•m-2•s-1, light saturation point (LSP) was 1467μmol•m-2•s-1, and its apparent quantum efficiency (AQE) was 0.024; (2) Under sunlight condition, the diurnal variation of photosynthesis showed double-peak curve, and the peak value reached at 10︰00 and 16︰00, respectively, which revealed midday depression. And correlation analysis showed that the factors mainly affecting photosynthesis were transpiration rate (Tr), stomatal conductance (Gs) and photosynthetically active radiation (PAR); (3) The diurnal variation of transpiration was close to single peak curve, and its mainly affecting factors were atmospheric temperature (Ta), leaf temperature (Tl) and photosynthetically active radiation (PAR); (4) the diurnal variation of water use efficiency (WUE) showed inverse single peak curve.