In this study, the effects of nucleus diameter on tensile-peel strength in electrical resistance spot welding were investigated. Taguchi design approach has been employed to the experimental planning, the L-27(3(13)) orthogonal array. The larger-the-better and the nominal-the-best response criteria were selected to obtain the optimum conditions for nucleus diameter and tensile-peel strength, respectively. In addition, the effects of five parameters, electrode pressure, clamping time, welding current, welding time, and holding time, were examined in this way. Results showed that the tensile-peel strength was related with nucleus diameter, and that welding current and welding time were the most effective parameters for improvement of the strength of weld joints.