Microstructure and mechanical properties of friction stir welded joint using an AZ31 magnesium alloy of 10 mm in thickness was investigated. The welding condition range of weldability was narrower than thin sheet such as 5 mm in thickness, but the defect was not recognized by the appearance and the macroscopic observation, and a sound joint was provided. The microstructure of stir zone were showed fine structures than a base metal was accepted. The grain size of center of stir zone were smaller with increase of the welding speed, and the defects such as kissing bond were not observed. The maximum tensile strength of joint showed 214 MPa at tool rotating speed 1610 rpm, welding speed at 0.5 mm/s, and the joint efficiency of 88％ of base metal. Impact value of joint put into the notch at weld center at room temperature showed maximum value of 124 kJ/m2 at tool rotating speed 1610 rpm, welding speed 3 mm/s. The back side notch and the AS notch, the value of impact strength of the RS notch, but the impact value which was equal to a base metal was provided both.