The present study aimed to explore learners' perceptions while using Google Translate (GT) during online English-Writing lessons and to evaluate its effectiveness on learners' English writing skills. Online questionnaires and individual interviews were used to collect self-reported opinions from 24 Vietnamese students. Learner writing samples from the 12-week online session were also collected in order to identify possible improvements in writing. The findings revealed that the learners generally reported a positive effect on their writing from using Google Translate. In contrast, textual analysis of learners' writing samples indicated that although there were slight improvements in their writing skills, problems still exist. According to an error analysis that was conducted, negative transfer of structures from the students' L1 (Vietnamese) accounted for most of the lexical and syntactic errors identified. The findings demonstrate that GT is a useful support tool for teaching English writing. The contrastive analysis in the present study contributes to language interference studies and discussions on bilingual and multilingual education in the Vietnamese context.