Frictional coupling at the plate interface will control seismicity and tectonics in subduction zones. In this study temperature structure and deformation in the accretionary prism in the Southwest Japan subduction zone are simulated using a two-dimensional isoviscous corner flow model. Using surface heat flow data the average interplate frictional stress and the effective viscosity of the prism are estimated to be 2~30 MPa and 1020~21 Pa⋅s, respectively. The low stress value is probably due to high pore pressures caused by dehydration reactions in the subducting crust. Estimated temperature of the landward limit of the seismogenic zone is 350~500 ¿C; when the frictional stress is considered, the temperature becomes ~100 ¿C higher than that of the onset of plasticity of granitic rocks (350 ¿C). The brittle-plastic transition of gabbroic rocks might control the depth extent of the seismogenic zone in lower crustal depths. Flow velocity at the bottom of the prism is estimated to be ~0.01 m/yr, indicating that ~25% of the conversion has been consumed by the deformation of the accretionary prism in this subduction zone. ¿ 1999 American Geophysical Union