This article reports on the implementation of a web-based video-annotation system that supports online peer-assessment activities in a nursing communication training scenario. A quasi-experimental design was applied to investigate the effects of using video annotation on communication skills and professional attitudes. The participants were fourth-year students from two classes at a nursing college in Taiwan. One class of 50 students served as the experimental group, who used the video-annotation tool we designed to add their comments to videos. The other class of 50 students served as the control group and used YouTube to add comments. Although YouTube also provides video-annotation features, these are not often used. Two rounds of peer-assessment activities indicated that the video-annotation tool notably enhanced nursing students' communication performance. Specifically, the tool was helpful in promoting students' development of communication skills, but not their professional attitudes. The students using the video-annotation tool provided more suggestions in their peer comments than those who did not use it. Moreover, video annotation resulted in closer agreement between peer and expert ratings of students' communication. The use of a video-annotation feature could improve the effectiveness of online peer assessment and thus promote student performance.