Writing skills are not easy to develop in English language learning, especially when online learning is being implemented amid the COVID-19 pandemic. In order to improve students' writing skills, as well as increase students' autonomy, peer assessment is often adopted. The article presents findings of the research conducted in late 2021 on students' perceptions towards the use of peer assessment in an online English writing class in Vietnam. The research applied a mixed method model with a questionnaire and an interview as the instruments. The questionnaire has 18 items divided into 4 parts: students' personal information, students' attitudes towards peer assessment, advantages of peer assessment, and disadvantages of peer assessment. The participants were 97 English as a Foreign Language (EFL) students who experienced one semester studying "Advanced Writing Skills" in online classes. The findings of the research revealed that students had negative attitudes towards the application of peer assessment to improve writing skills and they shared ideas of advantages and disadvantages of peer assessment. It is recommended that specific guidelines on how to do peer assessment with samples should be provided before teachers implement this teaching technique in class. In addition, in online classes, teachers need to be aware of making students familiar with peer assessment, as well as in the virtual environment.