An assessment was conducted of a web-based formative peer assessment system (WFPAS) emphasizing learners' metacognitive awareness for their performance in ill-structured tasks. Results indicate that the WFPAS group achieved significantly higher scores for metacognitive awareness and performance in ill-structured tasks than the traditional peer assessment group and that a traditional peer assessment group achieved higher scores for metacognitive awareness than a self-assessment group. In addition, the WFPAS group showed significantly higher scores in motivation than the traditional peer assessment group. Results are explained from the perspective of peer interaction and scaffolding. The potential challenges and implications of the WFPAS are discussed.