Friction Stir Welding (FSW) is a solid state welding method. Now, for joining the variety of materials, especially dissimilar metals, has many applications. This method has no restrictions on the fusion welding. In addition to its many advantages, including the joining of metals with different melting points. In this paper, a successfully joint between Al5050Aluminum alloys to AISI304 stainless steel was reported. Friction Stir Welding (FSW) process was used for joining these dissimilar materials. In friction stir welding many parameters such as tool rotational speed, feedrate, offset and pin profile were effective on microstructure and mechanical properties of weld nugget by. This paper is focused on the effect of tool rotational speed, feedrate and offset on tensile strength and micro-hardness. In addition, effect of annealing operations was investigated on microstructure and mechanical properties of the weld nugget. The elongation and tensile strength of the weld nugget were increased 100 and 9 percent respectively by the annealing process.