We investigated the microplasma that is generated in a plasma display pixel with an auxiliary electrode by using numerical simulation. In particular, the effect of the auxiliary electrode on the efficacy of the plasma display was analyzed. The voltage of the sustain electrode was changed from 190 to 230 V, and the voltage of the auxiliary electrode was changed from 0 to 40 V. As the voltage of the auxiliary electrode increased at a given sustain voltage, the discharge current decreased, and the delay time of the discharge increased. The average density of the charged particles and the Xe excited species showed a tendency identical to that of the discharge current. The vacuum ultraviolet output and the electrical power decreased, and the luminous efficacy increased as the voltage of the auxiliary electrode increased. In the case of Xe excitation efficacy, the peak value in the auxiliary period increased as the voltage of the auxiliary electrode increased, whereas the peak value in the sustain period was shown to be comparable to the variance of the voltage of the auxiliary electrode. The infrared intensity increased as the voltages of the sustain and auxiliary electrodes increased. In order to verify the effect of the auxiliary electrode, the time-spatial distributions of the electrons and excited species were also discussed.