This study aimed to develop a rubric to assess participation of students in online discussion environments. For this purpose, the study included 168 students who participated in a course offered online during the spring semester of the 2015-2016 academic year. Developed based on the literature, the rubric consists of two parts (Form and Content, and Number and Density) and seven criteria in total. "Form and Content" consists of congruity of the message in terms of subject, clarity of the message, original value of the message, interactional value of the message, and directing the subject; whereas, "Number and Density" consists of the number of messages and their density. Four different discussion subjects were presented to the students in an online discussion environment. The researchers analyzed the students' messages individually. Exploratory Factor Analysis was performed in order to obtain evidence for construct validity. After conducting the factor analysis, results showed that the first dimension, which is "form and content" sub-factor, is unidimensional. The "number and density" sub-factor was included in the graded scoring key based on the literature and expert opinion. The findings show that the graded scoring key is reliable and valid.