As the number of participants in online distance learning courses increases, peer assessment is becoming a popular strategy for evaluating open assignments and for breaking the social isolation surrounding distance education. Yet, the quality and characteristics of peer assessment in massive online courses has received little attention. Hence, this study was set to examine peer feedback quality and grading accuracy in a project-based course. The study applied a sequential exploratory mixed methods design. It included 339 participants who studied the same engineering course, but in three different modes: on-campus (n = 77), small private online course (n = 110), and massive open online course (MOOC) (n = 152). Content analysis of feedback comments identified four categories: reinforcement, statement, verification and elaboration, arranged in an ascending scale of cognitive ability. The findings indicated that the MOOC participants provided more feedback comments and volunteered to assess more projects than their counterparts did. However, the on-campus students provided higher quality feedback and their peer grading was better correlated with the grades assigned by the teaching assistants.