Language transfer is one of the most significant aspects of cross-linguistic influence. It can be divided into two types: positive and negative, the beneficial and detrimental use of one language’s acquired characteristics in another. This study aims to investigate the L1 (Chinese) influence on Hong Kong ESL learners’ acquisition of conjunctions and whether the use of L1 has an impact on this influence. An online questionnaire combining questions about students’ English learning experience and a grammatical judgement task was sent to a government-funded secondary school in Hong Kong and completed by 79 students. Evidence of both positive and negative transfer from Chinese to English was found by testing six conjunctions or conjunction pairs: although…but, because…so, not only…but also, either…or, neither…nor, and despite. The results suggest that Chinese is likely to have an impact on students’ acquisition of conjunctions, and the use of Chinese in English classes can worsen negative transfer and boost positive transfer. The degree of transfer might increase when more Chinese is used in class. To alleviate the impact of Chinese transfer and to provide more effective L2 teaching, monitoring the use of L1 and regular evaluation might be useful. Teachers can also adjust the amount of Chinese used according to the students’ English proficiency level.