As a continuation of our studies on the psychophysical bias in the judgment of vowel pitch (Chuang and Wang. ASA 60.1 and 62.1) and duration (Wanget al.ASA 60.1), this study was aimed at quantitative investigation of perceptual bias in the loudness dimension. The experimental design and the four vowels [i,e,u, and ɑ] were the same as previous studies with [ɑ]as the reference. The perceptual loudness bias differences were obtained by equalizing the syllabic speech power of the four test vowels. The mean loudness difference, based upon data gathered from 12 listeners, indicated that in reference to [ɑ], [i]was 7.1 dB, [e] was 4.5 dB, and [u]was 3.8 dB louder, respectively. These results give quantitative support to the studies of Ladefoged (1961) and Allen (1971) which showed qualitatively that with speech power normalization, the high vowels [i] and [u]were judged louder than the lower vowel [ɑ]. Furthermore, these perceptually obtained loudness differences are found to be inversely matched to the reported intensity difference of sustained vowels (Lehiste and Peterson, 1958). The negative correlation between the production intensity and the perceptual judgment of loudness in congruent with results reported in our previous pitch and duration studies. [Work supported by NSF grant BNS 76‐00017 and partially supported by NS 13274.]