The role of cross-linguistic influence in bilingual children’s development remains a matter of debate. Some researchers have proposed that simultaneous bilingual learners develop the linguistic systems of two languages in the same way as matched monolingual children do. Other researchers have argued that bilingual children show different developmental pathways. This study investigates cross-linguistic influence in the acquisition of relative clauses by English-Mandarin bilingual children in Singapore. The elicitation task included narration and interview tasks. Thirty-six primary school students aged 6 to 11 years completed the task in both English and Mandarin. The results reveal that the number of relative clauses increased with age in both languages. Participants had a preference for subject relatives over object relatives. The most frequent error type in Mandarin involves postnominal relative clauses, which have not been reported in monolingual children in the literature and thus can be treated as evidence of transfer from English. The findings of this study provide evidence for cross-linguistic influence in bilingual children’s speech.