The objective of this study was to develop an online report writing activity that was a constructive and cooperative learning process for a course on traditional general physics experiments. Wiki, a CMC authoring tool, was used to construct the writing platform. Fifty-eight undergraduate students (33 men and 25 women), working in randomly assigned groups of 2 to 3 members, participated in this course. Both quantitative and qualitative data, including assessments of reports, questionnaires, interviews, and records of discussion on wiki, were collected and analyzed to investigate the course design of online report writing. Results showed that students recalled, discussed, searched for, and integrated auxiliary information, reflected on the experiment, and shared meaning in the process of online writing. Evidence of positive interdependence, promotive interaction, individual accountability, social skills, and group processing proved that students worked cooperatively to accomplish shared learning goals. A higher average score of online writing than that of traditional paper writing indicated that students wrote online reports better, compared to the traditional method. Student participants and the instructor in the course responded positively when they were questioned on their perceptions of the Wiki-based report writing.