Flow around Savonius rotors was simulated by solving 2-d Navier-Stokes equations. The equations were discretized by finite volume method for space and fractional step method for time. Convection terms were specially discretized by an upwinding scheme for unstructured grid. Only rotating rotors were simulated in this report. The values of parameters were as follows : Reynolds number, 10⁵ ; overlap ratio, 0 and 0.16 ; tip speed ratio, 0.25-1.75. Results showed good agreement with experimental data for the following points : optimum tip speed ratio is 0.75-1.0 ; overlapping is effective in increasing power coefficient. Moreover, simulated flow fields showed that vortex shed-ding occurs not only at the tips of the bucket but also at the back of the bucket.