The current study investigated the effects of different computer-supported cooperative prewriting strategies (text-based brainstorming, drawing, and mind mapping) on the writing performance of elementary-school EFL (English as a foreign language) learners in Taiwan. Three intact classes of fifth graders (N = 81 students; 27 per prewriting strategy group) participated in this study. These subjects in different classes used different prewriting strategies at the prewriting stage. A quasi-experimental design was adopted in this study to collect experimental data, including the scores of English composition (two writing works), the level of grammar knowledge, and the level of English writing motivation. The analytical results indicated that in both the mind mapping group and the drawing group, the participants' English grammar knowledge had increased significantly after the treatment. It was also found that different computer-supported cooperative prewriting strategies benefited young EFL writers in different dimensions of English composition. The influence of using the mind mapping strategy seems most powerful on young EFL writers, followed by the drawing strategy and the text-based brainstorming strategy. As to English composition motivation, almost all of the participants appreciated the computer-supported cooperative prewriting activities and were also aware of how these activities benefited their English writing.