By using a portable PAM-2000 fluorometer and a spectrophotometer (722 Model), the diurnal variation in main chlorophyll fluorescence parameters such as initial fluorescence (F0), maximal fluorescence (Fm), ratio of variable fluorescence to maximal fluorescence (Fv/Fm), non-photochemical quenching (qN), fast-phase (qNf) and slow-phase (qNs) of non-photochemical quenching, electron transport rate (ETR), and relative content of xeaxanthin in Citrus unshiu leaves were observed on fine days. The results showed that ETR, qN, qNf, qNs and relative content of xeaxanthin increased with increasing light intensity, but Fm, F0, photo-chemical efficiency of PS II (Fv/Fm) decreased. After treated by DTT, qNs was much lower, but F0 was much higher than control. These results indicated that xanthophyll cycle and delta pH existed, and played an important protective role against photo-damage to the photosynthetic apparatus in Citrus unshiu leaves during diurnal variation of photosynthetic efficiency.