Table ADC-225-MK3 CO2 gas analyzer and PAM-2000 portable fluorescence measurement system were used to measure the net photosynthetic rate (Pn), initial fluorescence (Fo), maximal photochemical efficiency of PS II (Fv/Fm) and electron transport rate (ETR) of satsuma mandarin (Citrus unshiu Marc.) and navel orange (C. sinensis Osbeck) leaves. The results showed that exposure of citrus plants to high temperature (38-40 degrees C) led to a lowering of Pn, Fv/Fm, and ETR, whereas Fo increased. After exposure to high temperature for 25 days, compared with normal temperature (25 degrees C), the Pn of satsuma mandarin and navel orange leaves decreased by 55.6% and 39.8%, Fv/Fm decreased by 22.0% and 6.7% and ETR reduced by 55.0% and 41.5%, respectively. On the other hand, Fo increased 113.8% and 14.9%, respectively. With subsequent transfer to the 25 degrees C culture room for 10 days, Pn, Fv/Fm, Fo and ETR recovered significantly. These results demonstrated that the reduction of Pn in Citrus was related to the inactivation of PSII reaction center.