A psycho-acoustical experiment was conducted using synthetic vowel-like stimuli to examine effect of formant amplitude in vowel perception. Nine combinations of formant frequencies were examined. For each combination, relative amplitude of the third to the second formants was modified in seven degrees. In eight of the nine combinations, perceived vowels were changed according to the formant amplitude although every formant frequency kept constant. Furthermore, this amplitude effect was observed even when frequency separation of the neighboring formants was greater than 3.5 Bark. The result suggested that formant amplitude is effective cue for vowel perception as well as formant frequency.