Previous studies have found that but and so occur frequently in native and non-native English speakers’ speech and that they are easy to acquire by non-native English speakers. The current study compared ideational and pragmatic functions of but and so by native and non-native speakers of English. Data for the study were gathered using individual sociolinguistic interviews with five native English speakers and ten L1 Chinese speakers. The results suggest that even though the Chinese speakers of English acquired the ideational functions of but and so as well as the native English speakers, they underused the pragmatic functions of them. The findings indicate that there is still a gap between native and non-native English speakers in communicative competence in the use of but and so. The present study also suggests that speakers’ L1 (Mandarin Chinese) and overall oral proficiency in oral discourse affect their use of but and so.