The characteristics of zero-correlation phase plate smoothing interference speckles are analyzed theoretically and simulated numerically. The results show that although the zero-correlation phase plates are designed on the incident laser beam being a plane wave, the speckle contrasts of the far-field spots with these phase plates are much smaller than that with the random phase plates in the case of near-field phase and amplitude having small fluctuations. Therefore the zero-correlation phase plates are more effective than random phase plates for speckles smoothing.