Abstract: As an essential element, magnesium (Mg) plays a key role in plant development. Some reports have showed that Mg-deficiency leads to the decline of the rate of photosynthesis (Pn) followed by the decrease in the yield of crop. In order to detect physiological mechanism for lowering Pn by Mg-deficiency, the diurnal changes in gas exchange and chlorophyll fluorescence parameters in the fully expanded 2nd top leaves in a hybrid rice Shanyou 63, which grow in three nutrient solutions containing essential elements with 0 mmol/L Mg2+ (deficient) ,1.65 mmol/L Mg2+ (control) and 8.25 mmol/L Mg2+ (high) respectively, were characterized in this experiment with a portable photosynthesis system (LICOR-6400, USA) during a sunny day with significantly variable photon flux density (PFD) from about 100 mol/(m2s) at 6:00 a.m. and 18:00 p.m. to 1400 mol/(m2s) at noon. Antioxidant enzymes activities and contents of photosynthet ic pigments, soluble proteins and malonyldialdehyde (MDA) from the same leaves were also determined at same time. The results showed that, compared with the control plants, markedly lowered Pn indicated that an aggravated photoinhibi tion occurred in Mg-deficient plants. Chlorophyll content, electron transport rate (ETR) and CO2 assimilation were suppressed , the three major processes of photosynthetic reaction, which are primary reaction, proton electron transport and CO2 assimilation respectively, are all inhibited under Mg-deficiency conditions. At 13:00, qP decreased and qN increased in control plants, indicating that non-photochemical quenching dissipates excessive excitation energy to protect photosynthetic apparatus from high light damage. On the contrary, both qP and qN decreased in magnesium deficientplants, showing that Mg deficiency impairs excessive excitation energy dissipation. Since xanthin cycle which is one of the important channels of excess excitation dissipation dependents on pH gradient across thylakoid membrane and Mg makes great contribution to the formation of thylakoid-across pH gradient, it suggests that Mg-deficiency suppresses xanthin cycle to block the increase of qN. Judging from the lowering of F0, Fm, F0' , Fm' , Fs and Fv ' /Fm' of Mg deficient plants and the rising in SOD, POD and CAT activities as well MDA contents, we suggest that, though the light energy harvesting and transferring to PSⅡ reaction center has decreased for rice plant subjected to magnesium deficiency, excessive excitation energy is still accumulated under high light conditions, which triggers the overproduction of reaction oxygen species and induces the increase of activities of antioxidant enzymes. Large quantities of free radicals not to be scavenged by antioxidant enzymes lead to the aggravation of membrane lipid peroxidation to the extent that the leaves loss green even died. The results also showed no significant difference in all parameters tested from five-fold controlled Mg2+ content treatment.