This paper investigates foreign accent by comparing vowel production of native speakers, heritage and non-native learners with data from a large corpus of spontaneous Chinese learner speech. Snippets were evaluated by untrained Mandarin raters on accent ratings and followed up with acoustic analysis of vowel qualities. The rating result showed a high correlation between accent and pronunciation. It is found that it is easier to improve the goodness of pronunciation, but the impression of accent is hard to change. Duration and formant studies reveal that L1 transfer has long-term impact on accent. The vowel [u] produced by second language learners was more fronted than that by native speakers. The vowel [y] is difficult for learners who associate front vowel with unrounding or, alternatively, whose performance falls between [y] and [u]. The formant space of Mandarin low vowels [a] and [a] produced by learners were in the opposite direction from the way native speakers did or were not distinguished. The research findings have implication on language teaching and pronunciation training method.