This study investigates experimentally and numerically unsteady flow fields in an axial compressor operating at highflow rate in order to understand the flow structure in the stator row operating at windmill condition. The experimental and numerical data are compared by time- And phase-lock-averaged techniques. Additionally, unsteady vortex structure is investigated by numerical technique. At windmill condition, the incidence angle to the stator row becomes extremely negative. Therefore, large separation occurs near pressure surface in the stator passage. The experimental and numerical results indicate that a large vortex is generated in the separation area. According to the numerical results, part of the vortex migrates downstream, and the vortex produces blockage of the main stream of the stator passage. Therefore, net flow area in the stator passage becomes small so that the flow between the vortex and suction side of stator vane is accelerated. As a result, the total pressure deterioration is generated in the stator passage because the high speed flow and the vortex cause high shear.