As internal combustion engine in automobile operates much quieter than before, the noise emitted by claw-pole alternator becomes more non-negligible. To address this problem, the magnetic force, which caused acoustic noise, was analyzed. Force harmonic relevant to the noise peak was identified and set as an optimization object to weaken. The influence of each rotor structural parameters on the force harmonic and output current was analyzed, and the four most influential parameters were chosen as the optimization variables. With the genetic algorithm, an optimum rotor shape generating minimum force harmonic, without severely deteriorate the output performance of the alternator, was obtained. Calculated by the noise predicting model which was verified by experiment, acoustic noise produced by the optimized claw-pole alternator was 14.3 dB (A) less than the initial one, with the output current decreased within 5 percent.