Rheological parameters of Mongolian Oak in compression perpendicular to grain was determined by the method of Separating the rheological parame ters. The results indicated: the linear relationship between elastic deformation and constant compression stress existed in wide stress range; the instantaneous plastic deformation in different sections was separately dependent on the stress linearly; the behavior of retarded elasticity and plasticity depending on time was mainly experssed in terms of logarithmic function; the behavior of viscoelasticity consisting of elasticity and retarded elasticity was closely corresponding with linear viscoelasticity. The results were almost the same as the results obtained from Poplar. Therefore, the differences on rheological properties between Poplar and Mongolian Oak existed in quantitative aspect only. The ratio of the rheglogical parameters between Poplar and Mongolian Oak respectively is as follows: 1) 2.4—3.4: 1 for elastic compliance; 2) 1.8—2.7: 1 for retarded elasticity. The ratio of the critical stress between Poplar and Mong0Iian Oak in compression perpendicular to grain was 1: 2.