The bolt and nut is a representative fastener used in a machine assembly. When a load is repetitively applied to the bolt and nut assembly in the perpendicular direction of the bolt axis, the nut gradually loosens. To address this problem, nuts of diverse structures and shapes have been developed. In this study, a loose-proof locking nut is developed using a spring. The spring prevents the loosening of the nut by augmenting the axial load of the bolt. Three dimensional finite element models for the bolt, nut, spring and vibration plate are generated, and the loosening performance of the nut is analyzed using MSC/Marc, a commercial finite element analysis program. With the optimization technique based on the design of experiments, the shape and material of the spring are optimized. The optimal results are applied to the bolt and nut connection. The performance of the newly developed locking nut with a spring is compared to that of the general nut and bolt connection without a spring and the effectiveness of the locking nut is verified.