In this study we consider what phonological factors affect the formation of loanwords com-pound truncation (hereinafter called LCT) in Japanese. Many LCT in Japanese tend to be ab-breviated as quadrisyllabic words by clipping the initial two morae from each component of the baseword (e.g., dezitaru + kamera → dezikame ‘digital camera’). The LCT has been treated as evidence to claim the role of bimoraic foot and prosodic minimality in Japanese (Poser, 1984; Itô, 1990). However, these previous studies have conducted theoretical analyses exclu-sively on the mechanism of truncation form, leaving the problem that the explanation is not supported by experimental evidence.

To reveal what factors affect LCT formation, we conducted a forced-choice test focusing on the phonological factors of the truncation form. Participants were asked to choose the most natural form between a trisyllabic pattern and the quadrisyllabic pattern (e.g., muudo ‘Mood’ + konsaato ‘Concert’ → muukon (trisyllabic pattern) or mudokon (quadrisyllabic pattern)).

Results were as follows. First, Japanese native speakers tended to choose the trisyllabic pat-tern (e.g., muukon) as a more natural form. Second, the truncation form depended on the lenght of the first component in the compound words. Third, the length of the first component is closely related to the type of special mora formating a heavy syllable. Our findings are expected to provide considerable insights into a phonological and morphophonological study by exper-imentally verifying the phonological factors affecting truncation form.