This study investigated the perceptual effect of duration and F0 peak location on L1/L2 perception of English lexical stress. A nonsense bisyllabic English word embedded in a frame sentence, whose F0 was set to reach its peak after the word, was used as the stimuli of the perceptual experiment. Native English and Japanese speakers were asked to determine lexical stress locations. The results showed that in the perception of English lexical stress, F0 peaks that immediately followed the stimulus words perceptually affected the subjects in an opposite manner: Japanese speakers perceived these F0 peaks as a cue to lexical stress in the preceding syllable, while English speakers perceived them as an independent prominence peak and showed perceptual stress shift. The findings also confirmed the claim by previous studies that, while the perception of Japanese subjects is scarcely affected by duration, English subjects show great sensitivity to it.