The present study was performed to find efficient and useful length of voice sample for MDVP analysis, by investigating the correlation between perceptual GRBAS scales and objective MDVP measurements, with the five different lengths of voices of 20 patients with vocal polyp: 0.5, 1.0, 1.5, 2.0, 2.5 seconds. The results are following: (1) 1.5-second sample of MDVP showed the highest correlation between the perceptual judgement and objective measurement, and 1.0-second sample showed the lowest. The difference between the two samples was found in the number of the statistically significant correlated pairs of MDVP parameter-GRBAS scale. (2) The two extreme edges of the lengths, 0.5-second and 2.5 second showed no statistically significant difference.