Photosynthetic rate(P.), transpiration rate(T.), stoma conductance(G.), and light response curve of Potentilla d iscolor Bunge were measured in the field using an open gas exchange system, Li-6400 Porta ble Photosynthesis System in the hilly area of the Loess Pla tea u.Results showed that the diurnal va ria- tion of net photosynthetic rate presented a single-bottom curve.The maximal P.was 14.91 μmol/(m²?s) at 8:00 a.m and the minimum P0.13 μmol /(m²?s) at 14:00 p.m.Potentilla d iscolor Bunge got into the state of photosynthetic noon break. The light compensation point, the light saturation point, and apparent quant um efficiency of Potentilla discolor Bunge were 51.4, 527, and 0.0566 μmol/(m²? s),respectively.This indicated that Potentilla d iscolor Bunge was a typical sun plant.During the course of light response, with RPA increasing, T, rose and its diurnal variation was in agreement with G,.There was an exponent rela tionship between T, and G, The water use efficiency(WUE) of Potentilla d iscolor Bunge abided wit h a parabola model as RPA increased.At the beginning, WUE increased greatly and the maxim um appeared at 4.68 mmol/mol when RPA was 800 μmol/(m² ? s).Then WUE decreased with a less variation.