This study is intended to describe and analyze the durational patterns of native Japanese speakers learning English, with a focus on two major prosodic effects: phrase-final lengthening and stress-timed shortening. To investigate the relative contribution of these effects, a production experiment was conducted, adapting the methodological framework of Beckman and Edwards (1990). The effects of three degrees of boundary strength (the boundaries separating the members of a compound; two phonological phrases; and two intonational phrases) on the two phenomena were analyzed. Native English speakers, beginning Japanese learners of English, and advanced Japanese learners of English were compared.