Due to various difficulties in pronunciation, utterances by non-native speakers may be lacking in fluency. The Japanese pronunciation is said to have mora-synchronism, and, therefore, we assume that the disfluency may cause larger variations in vowel durations. Analyses of vowel (and CV) durations were conducted for Japanese sentence utterances by 2 non-Japanese speakers and one Japanese speaker (all female speakers). Larger variations were clearly observed in non-Japanese utterances. Then, 10 Japanese speakers were asked to rate the non-Japanese utterances. Strong negative correlations were observed between durational variations and pronunciation ratings. Based on the result, a method was developed for automatic evaluation of non- Japanese utterances. The ratings by the method were shown to be close to those by native speakers. Also, in order to offer a corrective feedback in learner's voice, non-Japanese utterances were modified in their vowel durations by referring to native Japanese utterances. The modification was done using TD-PSOLA scheme. The result of listening test indicated some improvements in nativeness.