Multi-objective optimal design of a vehicle power train is presented for the geometry of a torque converter. The power train is composed of an engine, a torque converter, a transmission and a final gear, and it is an essential subsystem of a vehicle in items of its acceleration performance and fuel economy, etc. In this paper, a practical method of optimizing the geometry of a torque converter is proposed, and trade-off relationships among the objectives are demonstrated by means of design optimization. In the optimization calculation the geometry is manipulated using spline curves, an appropriate evaluation method for torque converter performance is utilized, and successive quadratic programming is used as an optimization algorithm. A computational example demonstrates the effectiveness of the proposed optimization method.