Abstract: Fully coupled thermo-mechanical model was adopted to study the effect of pin shapes on deformations and temperature fields in friction stir welding.Results indicated that a flatter shoulder could lead to the decrease of temperature and plastic deformation.The efficient power decreased simultaneously.With comparison to the case of cylindrical pin,the temperature decreased in the case of conical pin.The decrease of the frictional heat induced by the decrease of the slipping rate was the reason for such a decrease of temperature.The plastic deformation increased when the conical pin was adopted.When the smaller shoulder was used,the temperature decreased obviously,which was caused by the decrease of the frictional area.So,higher angular velocity should be used when the smaller shoulder was used to improve the welding temperature and the weld quality.