A single-phase brushless dc motor requires an asymmetric air gap to eliminate the dead point where the excitation net torque is zero. In this paper, we use the finite-element method (FEM) to analyze the properties of cogging torque, starting torque, and electrical circuits. Others have discussed the influences of various asymmetric air gaps on these properties. However, the optimum asymmetric air-gap condition is very difficult to find. Here, we describe an optimum asymmetric air-gap structure that not only produces an optimum starting torque but also reduces the cogging torque to a tolerable range. We compare simulation results with the measurement results to illustrate the accuracy of the FEM model.