This research explores the phenomenon of interference, with a particular focus on the impact of first language interference on the English speech of native Mandarin speakers living in Makassar over a certain period of time. The author chose this aspect of bilingualism because of the frequent occurrence of English pronunciation errors among native Mandarin speakers. This study aims to investigate how Mandarin pronunciation interferes with the English pronunciation of people in Makassar. This research is limited, concentrating only on pronunciation interference when native Mandarin speakers speak English, using qualitative methods. Data collection was carried out using recording and interview techniques, with the author analyzing sentences that contained interference based on Weinreich's theory. The findings revealed that interference occurred in phonological rules that were poorly learned by native Mandarin speakers learning English, and some English syllables were absent in Mandarin. As a result, when speaking English, native Mandarin speakers transfer syllabic elements from their language, thereby causing additions, deletions, and modifications to the target language. The conclusion drawn is that interference is manifested in linguistic aspects, including the influence of first language habits, limited knowledge of English phonology, the impact of the mother tongue, and environmental factors. This highlights the large number of errors in bilingual speech, where sentences are disrupted due to the interference of their first language, Mandarin, resulting from native Mandarin speakers' lack of proficiency in English.