First language phonological categories strongly influence the late learn-ers’ perception of second language (L2) categories. In the present study, we ex-amined whether duration or vowel quality is more important for Mandarin Chineselearners of German to distinguish German tense and lax vowels compared to Ger-man native speakers. In order to have good control of the two factors, 70 Germanwords were synthesized with an articulatory speech synthesizer (VocalTractLab).For each of the 70 words, three “incorrectly pronounced” variants were generatedby manipulating the target vowel in terms ofduration(long vs. short),quality(tens-ing vs. laxing), orboth. Then 10 native German speakers, 16 Chinese advancedlearners, and 19 Chinese beginners were asked to listen to the synthesized and ma-nipulated words and identify the correctly pronounced words. It was found thatGerman listeners could distinguish tense-lax vowels with almost 100% accuracy,and the vowel quality seemed to be the primary cue. Chinese listeners achieved anaccuracy of 82% for advanced learners and 76% for beginners. Moreover, Chineselearners relied more on duration than quality in their identification. This suggeststhat durational cues in vowel perception are easier to learn even if the duration isnot a distinctive feature for vowels in their native language, while quality dimen-sions are more difficult for L2 learners to acquire. The findings can shed some lighton L2 speech acquisition.