A perceptual study has been performed to investigate relationship between acoustic parameters and the voice-individuality making use of a pitch synchronous analysis-synthesis system. Voice-individuality is involved in many acoustic parameters and the aim of this experiment is to examine how individual parameters affect the voice-individuality by separately giving them some distortions. Formant-frequency shift and bandwidth manipulations are given for spectral distortion, F_0-shift for source manipulation. As the waveform distortion, zero-crossing and center-clipping techniques are used. It has been found that formant-shift is very sensitive to voice-individuality change and F_0-shift and bandwidth manipulations are rather tolerant to the voice-individuality. The results of waveform manipulation reveal that the voice-individuality is kept more than the phonetic information for zero-crossing distortion and the results for center-clipping distortion are reverse.