There have been a number of studies on the effects of formative assessment on Learning Management System on EFL writing achievement. Nevertheless, in the context of Vietnam, little research has been done on education technology that enhances online formative assessment in teaching writing. In an attempt to further the previous research nationwide and worldwide, this study examined the impacts of formative assessment on students' writing achievement in a writing course of Learning Management System Schoology. A quasi-experimental design was carried out among two groups of fifty undergraduate Vietnamese students over a fifteen-week writing course using the main activity named online discussion board. The data analysis from the writing test scores suggested that there was statistically significant difference between treatment group and control group of participants. It is recommended that teachers should employ online formative assessment in teaching writing to help improve writing achievement among EFL learners in Vietnam and similar contexts.