Heat stress at the spikelet primordium differentiation stage of rice significantly decreases the grain yield due to reduced grain number per panicle. The hybrid rice Yongyou 12 was used as material to investigate the role of salicylic acid(SA) in alleviating the negative effect on spikelet differentiation induced by heat stress. The results indicated that under natural conditions, low concentration of SA increased the spikelet number per panicle, while high concentration decreased the number. Interestingly, either low or high concentrations of SA (10-50000 μmol/L) increased the grain number per panicle under heat stress. The grain yield and grain number per panicle peaked under the treatment of 100 μmol/L SA compared with their respective control (0 μmol/L) under both natural conditions and heat stress. This might attribute to the increase in the leaf photosynthesis rate, antioxidant enzyme activities, concentrations of auxin(IAA) and zeatin(ZR) in spikelets under heat stress. Heat stress occurred at spikelet differentiation stage seemed to cause little damage to the leaf, but the actual light quantum efficiency and net photosynthetic rate of leaf were increased when sprayed with SA. Accordingly, there was no significant difference in the activities of SOD, POD and CAT of leaf in rice. However, these antioxidant enzyme activities in spikelets were significantly decreased when subjected to heat stress, especially under the treatment of 0 μmol/L SA, suggesting that SA could enhance anti-oxidation. Further, SA could also prevent the decrease in the concentrations of IAA and ZR in spikelets induced by heat stress. In sum, SA obviously enhanced heat-resistance in rice at 100-1000 μmol/L.