The perception boundary between single and geminate stops was examined by regression analyses in 3- and 4-mora Japanese words spoken at various speaking rates. It was found that the perception boundary is well predicted by a linear function with duration of stop closure and durations of word or disyllable which contained the single and geminate stops. However, we conclude that the disyllable duration was a better variable than the word duration because it provides a more consistent explanation for the perception boundary regardless of word length and speaking rate variations. The results support a relational acoustic invariance theory.