Peer Assessment (PA) has recently been gaining increasing popularity in higher education due to its advantages in engaging students, and its use in English as a Foreign Language (EFL) writing classes is very common in Indonesia. The implementation of PA became more necessary when the government regulated emergency remote teaching in response to the COVID-19 crisis. This study examines the implementation of an online peer assessment (OPA) and the constraints encountered by the students in the OPA of a paragraph writing class. To achieve the objectives of this study, an embedded mixed method design was employed, which consists of an interview, observations, documentation, and a survey. The respondents in this study are a teacher of the writing class and 30 English Education Department students of a state university. The gleaned qualitative data were analyzed thematically while the quantitative data were analyzed by finding the percentages generated by Google Forms. The results show that the teacher made use of the three stages of the OPA: preparation, instruction, and evaluation. The students faced three constraints: the quality of the feedback, impartiality, and the clarity of the assessment criteria. It is recommended that when performing an OPA, teachers provide clear instructions regarding the stages and pay attention to the issues experienced by students.