The contributions of genre-based approach to writing have been proved in many studies in different contexts. However, empirical evidence of effects of genre-based instruction on Vietnamese EFL learners is rare. This article reports the results of an experimental study investigating the effects of genre-based approach on Vietnamese learners' ability in writing argumentative essays. Twenty EFL students at a college in a Mekong Delta province of Vietnam participated in the study. Participants were divided into two groups, labeled as group 1 and group 2; each group was taught how to write argumentative essays by using genre-based approach at two different points of time. Participants' ability in writing argumentative essays was measured by three writing tests: pretest, progress test, and post-test. Participants' perception towards the use of genre-based approach in learning argumentative essays was collected via semi-structured interviews. Results indicated that participants from both groups significantly improved their writing performance after the study. Results from the interviews showed that participants held a positive attitude towards the contributions of genre-based approach to their ability in writing argumentative essays.