Self-assessment is believed to complement peer assessment in the classroom. However, whether, how and why this is done remains unclear. This study, by investigating the combined use of self and peer assessment for an academic writing task among a group of undergraduate students in Hong Kong, aims to shed light on how self-assessment complements peer assessment. Self-assessment is found to complement peer assessment in five ways: (1) it guides students to revise when peer assessment is lacking; (2) self-assessment effectively supplements peer assessment when the students had access to peer assessment; (3) even if a student has access to quality peer assessment, self-assessment complements peer assessment due to the different reflection involved in the two processes; (4) self-assessment can supplement peer assessment in terms of issues related to social-affective burdens for the latter; (5) self-assessment also complements peer assessment in that it benefits high and low-achieving students. Two problems surfaced. One is the students' antipathy against self-assessment despite their overall positive perception about peer assessment; the other is the inadequacy of combining self with peer assessment in fostering learning outcomes regardless that the combined use of self and peer assessment helped the students with revision considerably.