【Objective】 Soil salinization is a soil problem getting more and more serious nowadays, reducing plant growth and affecting agricultural production. The area of salinized rice fields is expanding, what is more, rice (Oryza sativa L.), a main staple food crop of the country is quite sensitive to salt stress. More and more attention has been paid to the study on how to regulate salt tolerance of rice. Melatonin (MT) is an important class of indole compounds widely found in most organisms. More and more studies have found that though very low in content in plants, MT plays a very important role in physiological regulation and enhancement of plants. However, little has been found in literature on application of melatonin to rice. It is, therefore, of great significance to explore concentration-dependent effects of MT on salt tolerance of rice and its physiological mechanism. 【Method】A hydroponics experiment was carried out on effects of foliage spraying of MT to rice on plant height, biomass, root/shoot ratio, absorption and transportation of N, P, K and Na, K/Na ratio and S_K,Na (K/Na selectivity ratio). In this study, pass boxes were used for hydroponic cultivation of rice and extraneous MT sprayed. The experiment was designed to have 7 treatments, i.e. (1) Nutrient solution + spraying deionized water (Control); (2) Nutrient solution + 75 mmol L^-1 NaCl + spraying deionized water (S75); (3) Nutrient solution + 75 mmol L^-1 NaCl + spraying 25 μmol L^-1 MT (S75 + MT1); (4) Nutrient solution + 75 mmol L^-1 NaCl + spraying 50 μmol L^-1 MT (S75 + MT2); (5) Nutrient solution + 75 mmol L^-1 NaCl + spraying 100 μmol L^-1 MT (S75 + MT3); (6) Nutrient solution +75 mmol L^-1 NaCl + spraying 200 μmol L^-1 MT (S75 + MT4); (7) Nutrient solution +75 mmol L^-1 NaCl + spraying 400 μmol L^-1 MT (S75 + MT5). Each treatment had six boxes as duplicate, and solutions in the boxes were replaced every other day during the whole culture period; foliar spraying of deionized water or MT varying in concentrations in line with the treatments, in late afternoon every other day; and samples of plants collected for analysis on D15. 【Result】Results show as follows. (1) MT efficiently increased plant height and biomass (dry weight) of the plants under salt stress and reduced their root/shoot ratio. The effects intensified with rising MT concentration, and peaked in the treatment 200 μmol L^-1 MT in spraying rate, which was, 51% and 130% higher than the treatment 75 mmol L^-1 in MT spraying rate, respectively, in plant height and biomass. (2) In plants under salt stress, extraneous MT significantly increased N, P and K contents in shoot and root of the seedlings. The effects also peaked in the treatment, 200 μmol L^-1 MT in spraying rate. Compared with control, the treatment 75 mmol L^-1 NaCl in salt stress was lower in N translocation factor (N-TF), unchanged in P-TF, and higher in K-TF, however, foliar spraying of MT significantly increased all the three indices in plants under salt stress. But with MT spraying rate rising up to 200 and 400 μmol L^-1, N-TF increased, P-TF remained almost unchanged, and K-TF decreased significantly. (3) With rising MT spraying rate, Na content in the plants decreased significantly. Under salt stress, seedlings were high in Na-TF, which was not affected by spraying of MT, no matter how much. (4) Spraying of MT significantly increased K/Na in both shoot and root of the seedlings under stress, and the value peaked in the treatment 200 μmol L^-1 in MT spraying rate. Spraying of MT increased absorptive SK,Na (AS_K,Na) of the plants under salt stress. AS_K,Na continued to increase with rising MT spraying rate, and also peaked at 200 μmol L^-1 MT. Spraying of MT decreased transport SK,Na (TS_K,Na) of the plants under salt stress, especially when the rate reached 200 and 400 μmol L^-1. 【Conclusion】 To sum up, foliage spraying of 25 ~ 400 μmol L^-1 MT significantly increases NPK uptake, decreases Na accumulation, improves ion homeostasis in rice seedlings under salt stress, and promotes salt tolerance of the rice plants. All the findings demonstrate that foliar spraying of MT at 200 μmol L^-1 is the most appropriate practice to improve salt tolerance of rice.