Abstract: It was found that the value of instantaneous plastic deformation of poplar in compression perpendicular to grain is not proportional to the value of constant stress under the conditions of a wide range of the constant stress, various temperature, moisture content, and the specimen with or without water vapour passing through them, because there exist two turning points in general. The beginning point of the time-dependent plastic deformation makes considerable influence on the mechanical behaviour in a short time. When the condition of the specimen falls in the range before the first turning point, the viscoelastic deformation would be much more considerable than the plastic deformation in a short time because of the slight instantaneous plastic deformation and the later beginning point of the timedependent plastic deformation.