This paper proposes a new sustain waveform to improve the luminous efficiency of an alternate current plasma display panel (AC-PDP). The new sustain waveform is a superimposed waveform, which adds a ramp-waveform to a square-waveform, and has an increasing voltage slope between the rising and falling edge. This waveform can induce a longer-sustained discharge at the rising edge plus a self-erasing discharge at the falling edge, thereby improving the luminous efficiency. When compared with the conventional square sustain waveform, the proposed sustain waveform with a 9.3 V/mus voltage slope achieved a 65% higher luminous efficiency in a 4-in AC-PDP test panel even at a low frequency (62 KHz).