This study implemented an online peer assessment activity to help 52 Taiwanese in-service science and mathematics teachers to develop research proposals in an educational research course. The participants were divided into 16 groups, and each group was required to submit a proposal via three rounds of online peer-assessment activity. This study found that, through the online peer-assessment activity, the in-service teachers could enhance their proposals at the initial stage of the activity, but there was almost no substantial improvement at the later stage. In addition, the amount of feedback they obtained was positively related to the performance from the first round to the second round; however, few significant relationships were found from the second to the third round. This study also showed that the process of peer assessment could be used to clarify assessment criteria and that sufficient support from the instructor can maintain participants' attitudes towards this activity, which were relatively optimistic throughout. (Contains 1 figure and 8 tables.)