The talker dependency of the correct response rate (CRR) on English /r/-/l/ identification by Japanese listeners has already been reported. This paper shows that the talker dependency of the CRR can be explained by the acoustical dissimilarity (ADS) between an /r/ and an /I/ measured by the dynamic-cepstrum. The dynamic-cepstrum is a new spectral representation which simulates time-frequency forward masking. Nine spectral representations including weighted-cepstrum, mel-cepstrum, and delta-cepstrum were compared in terms of correlation between the CRR and the ADS. The ADS measured by the dynamic-cepstrum showed the best correlation with the CRR. The experimental results imply that Japanese listeners tend to identify /r/ or /I/ using the succeeding vowels affected by co-articulations.