Abstract ： In the field condition, taking the different pecan varieties as the samples which growing in cutting orchard in Lengshuitan, Yongzhou, Hunan. We studied the diurnal variation of fluorescence parameters.The results showed that minimal fluorescence（Fo）, maximal fluorescence
（ Fm）, optimal photochemical efficiency of photosystem II （Fv/ Fm）, effective quantum yield
of photosystem II（Yield）, photochemical quenching coefficient（qP）, non–photochemical
quenching coefficient（ qN）and apparent photosynthetic electron transport rate(ETR) all exsited obvious diurnal variation . The declines in Fm, Fv/ Fm, Yield and qP were observed at midday strong sunlight. On the other hand , qN amounted to maximum value at midday. Fo decreased and some varieties went up again in the evening, but the value of evening was lower than morning. ETR varied as a two-peaked curve. Fv/ Fm, Yield,ETR, qP, qN all showed difference responded to light intensity and temperature among different pecan varieties ,it can be used as the trait tolerant to photoinhibition.