Abstract: A series of friction stir welding (FSW) experiments were designed to measure and investigate the welding temperature field, mechanical loads of stir tool, residual stress and residual distortion. The measurements show that the accuracy of temperature data will be influenced by sampling frequency and the method of thermocouple set-up. The mechanical loads of stir tool mainly appear as down force and torque, which keep as constant during the period of quasi-steady welding, but their values depend on welding parameters. The main residual stress of FSW joint displays as longitudinal tensile stress, which distributes in welding zone and has a character of asymmetric double hump. The residual distortion of sheet after FSW has two opposite pattern of saddle state and anti-saddle state, and the distortion magnitude influenced by sheet dimension, rotation speed and welding speed.