In this study, the weldability of lower strength St 37 grade steel plate with a higher strength steel plate (i. e., St 44) using the friction stir welding technique has been studied. St 37 grade steel sheets are widely used in various industries. However, the joining of St 37 steel plates with higher strength grades such as St 44 grade steel is usually required when the strength of St 37 grade is not sufficient. Therefore, microstructural and mechanical properties of friction stir welded St 37-St 44 butt joints were investigated by conducting optical microscopy, microhardness measurements, and tensile testing. A fine-grained recrystallized microstructure was observed in the weld regions of the joints produced. The highest tensile strength, i. e. 385 MPa, was exhibited by the joint produced at 630 rev x min(-1).