As well known, the friction stir welding is a novel welding process which is a solid state welding process for sheet or plate using the friction stir phenomenon. This paper describes the effect of welding condition such as the rotation speed and the travelling speed during the friction stir welding process on the micro Virkers hardness and the microstructure of friction stir welded joints in AI-7075-T651 plate. From those investigations, the highest hardness of stir zone was observed at the welding condition of SO-3. The microstructures of the friction stir welded joints was not dependent on the welding conditions, but in the SO-4 specimen, the friction stir welding defect like tunnel shape was found in stir zone.