Change of pitch is a common disguise type adopted by criminals in forensic voice comparison, which introduces substantial variance of acoustic properties and results in poorer performance of speaker recognition. This paper investigates the acoustic properties of disguised voices with raised and lowered pitch from 11 Chinese male speakers. Parameters including fundamental frequency, syllable duration, intensity, vowel formant frequencies, and long term average spectrum (LTAS) were measured and statistically compared with those of normal voice. The effect of voice disguise on speaker recognition by both human and machine is also evaluated. The results show that speakers have different ability of adjusting pitch. Pitch change results in corresponding change of other parameters and degradation of speaker recognition by parameter discrimination, auditory perception and automatic speaker recognition, but some systematic changes of parameters provide clues for forensic voice comparison.