Argumentation and metacognitive reflection are required for effective thinking and convincing argumentation in engineering co-design. This study investigated engineering students' argumentation and metacognitive reflection in their final group reports and their correlation with the quality of their work in co-design. The groups practiced and gained experience in ideation in the product development process during academy-industry collaboration. Iterative theoretical content analysis and argumentation models were integrated to create a framework for identifying acceptable argumentation. The results suggest that the groups co-constructed acceptable arguments containing claims, reasons, and warrants with references. Meanwhile, the frequency of counterarguments, conclusions in advanced argument structures, and metacognitive reflection in the reports was low. The study suggests that instruction of more developed argumentation models and metacognitive reflection must be carefully integrated into engineering design education to foster convincing, advanced argumentation.