This study aims to investigate the patterns and the quality of online interaction during project-based learning (PjBL) on both micro and macro levels. To achieve this purpose, PjBL was implemented with online group activities in an undergraduate course. Social network analysis (SNA) and content analysis were employed to analyze online interaction during project work. According to the SNA results generated from the online discussion boards, the group cohesiveness of seven teams, indicated by density indices, varied considerably, from as low as 9.81 to as high as 30.00. Regarding the content analysis of two teams with high project scores (Teams F and G), team members not only shared information (Phase I), but also identified the areas of disagreement and clarified the goals and strategies (Phase II). They also conducted some negotiations (Phase III). However, team members with low project scores (Teams C and E) shared information and stated their opinions in most cases (Phase I), with not much social construction in the higher level. Although both Team C and G showed high level of group cohesiveness among the seven teams, it is notable that the high-performing Team G dedicated nearly 39.3 percent of online discussion to negotiating and co-constructing knowledge, contrary to the 5.9 percent of low-performing Team C. Based upon the findings, some implications were proposed for further research. (Contains 4 tables and 1 figure.)