Friction stir welding of a low carbon thin steel plate was performed with preheating the base material to increase the joining performance. The rotational speed of the tool was set from 1500 to 2000 rpm with a constant travel speed of 15 mm/min and preheating temperature of 50 to 150 °C. The tensile strength of 340 MPa was achieved for the preheated specimen compared with 310 MPa for the non-preheating specimen at the welding speed of 15 mm/min. Electron microscopic images of the preheated joint revealed a remarkable plasticized material flow and dynamic recrystallization, which resulted in the grain refinement that had increased the joining strength. The weld thinning issue was almost eliminated in the preheated welded specimen. However, by increasing the preheat temperature further, the tensile strength decreases due to the formation of defects from excessive heat. The preheated sample fractured at the base metal, revealing a ductile fracture mode.