The friction stir welding <tt>(</tt>FSW<tt>) </tt>of 5052-O and 5052-H34 aluminum alloys were performed to investigate the mechanical properties of the joints. The mechanical properties of the joints were evaluated by tensile tests and Vickers hardness distribution. The experimental results showed that tensile properties of the joints are significantly affected by the welding process parameters, such as welding speed and rotation speed. It was found that the joint strength of A5052-O joints could be made approximately equal to the strength of the parent metal by using a wide rotational speed and high welding speed. On the other hand, the joint strength of A5052-H34 joints was lower than that of the parent metal. When the welding speed was lower, the joint strength was the same as A5052-O joint. However, when the welding speed was increased, the joint strength was decreased rapidly. Therefore, the effect of friction stir welding on the heat treatment of 5052 aluminum alloys was clarified.