The surface and sectional microstructure of cut tobacco was observed by SEM (Scanning Electron Microscope) before and after expanding, and image processing of the sectional microstructure photographs of cut tobacco expanded at different expansion rates were carried out. The results showed that: 1) the structure of cut tobacco before expansion was tight and with recessed and wrinkled epidermal cells, while that of expanded cut tobacco was open and with protruding epidermal cells of smooth surface and less wrinkles; 2) with the increase of expansion rates, the number of pore <4µm in diameter and the area occupied by pores <6µm in diameter decreased, while the number of pore >4 µm in diameter and the area occupied by pores >6 µm in diameter increased; 3) for CO₂ expanded cut tobacco, there was a significant linear relationship between expansion rate and the number of and area occupied by pore whose diameter was <2 µm; and for KECHUANG expanded cut tobacco, there was a significant linear relationship between expansion rate and the number of and area occupied by pore whose diameter was 2-4µm.