Background and aim: In physical evaluations, measuring the joint angle is one of the most important outcomes to assess the patient's progress, the effectiveness of various treatment techniques and to delineate the need for future treatments. The aim of this study is to study the reliability of a system consists of skin markers, digital photography, and AutoCAD software for measuring the knee joint angles.

 Materials and methods: In first section, reliability of markers placement was evaluated within three different intervals (five minutes, two days, and twenty days). At second section, reliability of digital photography was evaluated while position of the skin markers and subjects was the same. At the end, in third section, reliability of angle measurement by means of AutoCAD software was assessed.

Results: In first section, the ICC for test-retest reliability of markers placement between first and second sessions, first and third sessions, and also first and forth sessions were 0.97, 0.94, and 0.73, respectively. In second section, the ICC for test-retest reliability of digital photography was 0.99 and in third section, the ICC for intra-rater reliability of angle measurement by means of AutoCAD software was 0.99 (p=0.05).

Conclusion: It was noted that the introduced system was highly reliable, so it could be utilized to measure the knee joint angles.