In the friction stir welding (FSW) process, a high-speed rotating tool, which consists of the probe and the shoulder, are employed to plunge into the faying surfaces. By using the friction heating and the stirring action of the material, the solid-state welding is accomplished to joint two pieces of metal by material diffusion to form a densification structure in the weld. According to the experimental results, the mechanism of friction stir welding is as follows: the probe plunge into the sample and the shoulder is in contact with the sample to generate a large amount of friction heat, which causes the materials soft. When the probe moves forward, the soft materials in front of the probe are scratched. The scratched materials are subjected to the rotational and squeeze actions of the shoulder so that they are refilled into the welded surface behind the probe. For the dissimilar metals joint (6061-T6 aluminum and C1100 copper plates), results show that when C1100 copper is located at the advancing side, the measured feed force appears drastic changes and it is also seen that the components of the force for the friction-stir welding of dissimilar metals become more unstable than those for the same metals joint, so that the structure which has been observed by optical microscopy appears to be open with pores and defects so that the welded quality becomes poor. According to the three components of the measured force during FSW process, the friction between the probe and the work piece can be computed. By using the friction theory, the hardness and the yield strength of the materials in front of the probe can be calculated, and then the faying surface temperature is approximately predicted to be 565.5 oC.