This study demonstrates the nonword repetition format comparable to other conventional tasks (picture-naming, reading, and so on) as a plausible measure of linguistic competence for adults with language attrition. Taiwanese speakers with and without attrition symptoms, defined by frequency of use, were recruited, and so were American learners of Mandarin Chinese. The results show that (1) fluent speakers' repetition accuracy of Taiwanese tones is significantly higher than attrition speakers', and American learners' is the worst, (2) among five target tones (high-level, low-rising, low-falling, high-falling and mid-level), the repetition accuracy of high-falling tone is the highest, and that of low-level tone is the lowest in non-word-final position across the three participant groups, and (3) the least accurate mid-level tone tends to be mispronounced as low-rising. The findings suggest that the participants' frequency of use and exposure to Taiwanese is positively correlated with the repetition accuracy, and mid-level tone is the most difficult category to learn. More crucially, the percent accuracy and confusion matrix of nonword repetition enlighten how midlevel tone is more susceptible to sound change.