Abstract: 【Objective】 The relationship between moisture diffusion ability and mobility of harvested upper tobacco leaf was investigated to provide theoretical basis for revealing their mechanism of moisture migration and diffusion during drying. 【Method】 The upper tobacco leaf was used as experimental material. The drying features，moisture diffusion coefficient of tobacco leaf，blade and main vein were investigated by drying experiments with different temperatures（35， 40，45，50，55，60 and 65 ℃），and the moisture state changes of blade and main vein were detected by low field nuclear magnetic resonance. 【Result】 The moisture content of tobacco leaf，blade and main vein decreased rapidly in the early stage of drying and decreased slowly in the later stage of drying，all showing typical features of falling rate drying. The drying rate of blade was larger than that of main vein. The moisture diffusion coefficients of tobacco leaf，blade and main vein all increased with the increase of temperature and moisture content，and the moisture diffusion ability of main vein was much larger than that of blade. As the drying progressed，the relaxation time of free water in both blade and main vein gradually reduced，indicating a weakened water mobility. However，the mobility of free water in main vein was stronger than that in blade. The free water content in both blade and main vein was at a high level at the beginning of drying. The free water content reduced synchronously as the moisture content decreased，indicating that the loss of free water dominated the drying blade and main vein，while the semi-bound water in blade and main vein only decreased greatly in the middle or late stages. The moisture diffusion coefficients of blade and main vein were extremely significantly positively correlated with the moisture content and relaxation time of free water（P<0.01），which suggested that the mobility of free water could be used to evaluate the moisture diffusion ability of blades and main veins. 【Conclusion】 The drying of harvested tobacco leaf，blade and main vein is controlled by internal moisture diffusion，and their moisture diffusion ability mainly relate to temperature，moisture content and mobility of free water.