In this present study, microstructural and mechanical properties of dissimilar jointing between austenitic stainless steel AISI321 and simple carbon steel St 52by friction stir welding has been investigated. The welding was carried out with rotational speed of 530, 710, 900 rpm and linear speed of 50, 80 mm/min. The microstructure was characterized scanning electron microscopy ( SEM ) and optical microscopy. Possible phase transformation were determined using X-ray diffraction technique. Tensile test, shear punch test and microhardness measurements were conducted to evaluate the mechanical properties of the jonts. The results showed that using friction stir welding, joints without defects were produced. In addition, no carbide and brittle phase were detected at the joining boundary. Microstructural investigation showed that significant decrease in grain siz was occurred in the stir zone of the austenitic stainless steel AISI321 for sample weld at rotational speed of 710 rpm and linear speed of 80 mm/min. Tensile test results demonstrated that failure was occurred in the weld metal for sample welded at rotational speed of 400 rpm for all linear speeds. Wherease, failure was occurred in the St 52 base metal for samples welded at rotational speeds of 710, 900rpm and linear speeds of 50, 80 mm/min. According to the shear punch test, the highest ultimate shear strength as well as yield shear strength was achieved for sample welded at rotational speed of 710 rpm and linear speed of 80 mm/min, while the mentioned sample showed the least elongation. In addition, the highest microhardness was measured in the stir zone of austenitic stainless steel sample welded with the above mentioned welding condition. Keywords: Friction stir welding, Austenitic stainless steel AISI321, Simpale carbon steel St 52, Mechanical properties, Dissimilar joint