In this study, the fundamental mechanism of friction stir welding was investigated to establish the relationship among the three components of the forces acting on the work pieces, the variation of the stirring energy, and the joint characteristics of the materials. A dynamometer designed by Chiou et al., was used to measure the axial force (z-direction), the feed force (x-direction), and the clamping force (y-direction). The output energy of servo motor was monitored by power meter. Experimental results show that with increasing welding speed, the feed force increases obviously, the axial force increases slightly, and the energy almost remains constant for the fixed rotation speed of the spindle. At the rotation speed of spindle of 800 rpm, the spindle angle of 1¢X, the pre-clamping force of 2kN and the welding speed of 60 mm/min, results show that the feed force is about 1kN when the probe is plunged into the specimens but the shoulder does not be in contact with the surface of the specimen. However, when the probe is plunges into the specimens entirely and the shoulder is in contact with the surface slightly, the feed force is reduced to 0.48kN. Moreover, when the shoulder is in contact with the surface heavily, the feed force is reduced to 0.2kN. This result indicates that the contact force between the shoulder and the specimen causes the material to become soft and to backfill into the weld, and then decreases the feed force. After the specimen of the 6061-T6 aluminum has been welding, the micro hardness measurements are made. Results show that the distribution of the hardness is quite consistent along the welding as the feed force approaches to 0.2kN. Furthermore, the appearance on the surface of the weld is quite fine, and thereby it is able to get the high and uniform quality. The spacing distance of the weld surface can be theoretically analyzed. It is found that the spacing distance increases with welding speed and decreases with rotation speed of spindle. The theoretical predictions are in very good agreement with the experimental measurements.