Random pulse width modulation (RPWM) techniques for inverters and converters can improve the spectrum of the output voltage and the emitted noise. Looking up a given RPWM table in memory is the most economic and convenient way to digitally implement RPWM strategy. However, the above way is not truly random but deterministic for the given RPWM table and repeated showing order. From our analysis, we can find that the different showing orders for the same elements of RPWM tables result in the different voltage spectrum. In this paper, the showing orders will be categorized and the optimal showing order corresponding to the optimal voltage spectrum will be found through the proposed index with the help of the personal computer.