This paper deals with the strengthening of spot welded lap joints by new hardened zone. Based on the equation to predict the strength proposed by past researchers, the factors necessary for the strengthening were extracted. And, based on these factors, the strengthening of spot welded lap joints was examined. First of all, in order to increase the hardened zone, the vicinity of the nugget edge was heated with a laser. Next, tensile shear tests were conducted by using spot welded lap joints which were heated by laser at various distances from nugget. Finally, the relationship between tensile shear strength and the distance from nugget center to the crack was clarified. As a result, by increasing the hardened zone, we found that the fracture position was far away from the nugget edge. And, we found that the tensile shear strength could be improved by 15%.