This study primarily aimed to identify difficulties in the English argumentative essay writing encountered by Vietnamese EFL English-major students. The study was conducted with 21 EFL English major students at Soc Trang Teachers’ Training College, Vietnam. To find out the challenges students faced when writing argumentative essays, a questionnaire with 29 items was designed based on the Likert scale to collect data from students. In addition, six participants were then asked to sit for the intensive interview to gain more in-depth difficulties in writing argumentative essays and the learning strategies they used to help improve their writing skill themselves. Qualitative data were analyzed through grounded theory, in which the data were transcribed and coded thematically. The findings from the students’ questionnaire and interviews revealed that they faced the following specific writing difficulties; unfamiliarity with argumentative rhetorical features, insufficient knowledge about grammar rule, insufficient academic vocabulary, difficulty in writing a clear thesis statement, and inability to provide solid evidence, to generate well organized ideas, and to write valid conclusions. The results also showed that most of the investigated students had difficulties in writing argumentative essays. Besides, they offered some ways to promote writing effectively. It is important to note that this issue demands a new and effective teaching essay writing strategy to achieve successful outcomes of teaching and learning process especially in essay writing context.