To study mechanism of tectonic deformation of coal, three samples of different ranks were tested under conditions of T = 300-700°C, P c = 400-600 MPa, = 10% -30% and = 3.63 10 - 6 -2.59 10 - 5 s - 1 . The characteristics of stress-strain curves and deformation structures, especially microstructure of samples indicate that: 1) plastic deformation of coal gradually decreases with the increase of coal rank and disappears gradually; 2) the presence of gas causes obvious decrease in coal strength and has a very important effect on the mechanical properties of coal; 3) a great quantity of gas in coal can not only a decrease coal strength but also can pulverize the coal, thus effecting rheology of coal.