This study analyzed the factors that affect student participation in discussion forum under the two main purposes. The first purpose was to examine the relationship between the students' individual demographics and categories of students' participation level (inactive, moderate, and active) in discussion forum of an online course. The second purpose was to examine the students' views about reasons for low level of interaction in discussion forum. A total of 196 students who attended computers systems and structures course of online certificate program were included in the study. The data was collected at the beginning and at the end of the course through online survey and semi-structured interviews. The descriptive and inferential statistical techniques were used to analyze the quantitative data. The content analysis method was used to analyze the qualitative data. The results of the study indicated that three student characteristics (achievement, gender and weekly hours of Internet use) showed a significant relationship with students' participation level in discussion forum of the online course. Also, the findings emphasized some of the critical issues that should be taken into account in designing online discussions, such as, students' workload and responsibilities, progress of interaction over the Internet taking more time, planned and structured instructional activities in discussion forum.