The field of English language education has seen developments in writing pedagogy, moving from product-based to process-based and then to genre-based approaches. In Vietnam, teaching secondary school students how to write in English is still lagging behind these growing developments. Product-based approach is commonly seen in English writing classes in Vietnamese secondary schools. Within that context of teaching and learning writing, this paper outlines the impact of our implementation of the process-based approach in English writing classes with Vietnamese secondary learners. The study following a two-group pre-test and post-test design aimed to test the effects of the process-based approach on learners' English writing and to investigate their perceptions of the learning process under this approach. Participants were 57 non-English major grade 10 students in an upper-secondary school in Vietnam's Mekong Delta. Participants were involved in a sixteen-week intervention program with eight forty-five-minute writing sessions every two weeks. Three instruments were used to collect data: the questionnaire on participants' perceptions toward the writing process, the English writing tests, and the interview on participants' perceptions. Results indicated a significant difference in learners' writing performance and their perceptions toward the writing process between the two groups after the study: Participants in the experimental condition outperformed those in the control condition.