The purpose of this study is to develop a standardized measurement tool that can be used to determine the participation styles of individuals participating in online instructional discussions. The scale consists of two dimensions called 'Why' and 'How.' The Why dimension comprises the main motivators of the participants' participation in online instructional environments, whereas the How dimension comprises items expressing participation behaviors or forms. Two separate datasets were used for exploratory factor analysis (450 participants) and confirmatory factor analysis (693 participants), and the scale was applied to both undergraduate and graduate students. The scale consisted of eight factors, with four in the Why dimension and four in the How dimension. The results of confirmatory factor analysis showed that the scale was able to identify four different patterns of participation. Expressed as participation styles, these patterns are To Socialize/Connective, To Get Information/Analytical, To Discuss/Innovative, and To Fulfill Requirements/Practical. According to these results, the Participation Style Scale for Online Instructional Discussions was assumed to be a valid and a reliable measurement tool for determining the participation styles of participants of online instructional discussions. These four styles are thought to contribute to the instructors, the researchers, and the learners who want to benefit from effective learning in online environments. Instructors and researchers can determine learners' participation styles before organizing discussion environments, and learners who think and gain awareness about their own participation styles can manage their discussion and learning processes more effectively.