It is rare for a component-member of a structure to be subjected to a simple stress state. Usually, it is subjected to a multi-axial stress state in many cases. Therefore, in order to more efficiently design a structure, it is necessary to fully understand the mechanical properties of constituent materials under such a state. In this report, the effect of combined axial force and torque loading on the elastic behavior of wood (Japanese beech and Japanese cypress) was examined. As the elastic behavior, the initial slopes of the stress-strain relationships obtained from combined loading tests are estimated. The specimen had a rectangular cross section with one of its major axes lying in the fiber (longitudinal) direction. The axial force and torque were applied in the fiber direction (along L) and about an axis lying in the L direction, respectively. Combined loading tests were performed using the proportional deformation loading method and the initial constant loading method. The results obtained were summarized as follows: (1) The effect of differences in loading methods on the relationships between shear stiffnesses and the states of combined stresses was confirmed, in particular, for Japanese cypress. (2) Differences in axial stiffness were observed between the two species under compression-shear combined stress state. While the axial stiffness of Japanese beech was not affected under the combined stress state, that of Japanese cypress tended to increase under compression-shear combined stress state. (3) The difference in shear or axial stiffness between the two planes was considered to be almost constant; however, when the axial or shear stress component of the combined stresses became dominant, the difference between the two planes tended to show a larger variation. (C) 2003 Kluwer Academic Publishers.