[Purpose] The aims of this study were to evaluate the type and extent of error in the measurement of range of motion and to evaluate the effect of evaluators' proficiency level on measurement error. [Subjects and Methods] The participants were 45 university students, in different years of their physical therapy education, and 21 physical therapists, with up to three years of clinical experience in a general hospital. Range of motion of right knee flexion was measured using a universal goniometer. An electrogoniometer attached to the right knee and hidden from the view of the participants was used as the criterion to evaluate error in measurement using the universal goniometer. The type and magnitude of error were evaluated using the Bland-Altman method. [Results] Measurements with the universal goniometer were not influenced by systematic bias. The extent of random error in measurement decreased as the level of proficiency and clinical experience increased. [Conclusion] Measurements of range of motion obtained using a universal goniometer are influenced by random errors, with the extent of error being a factor of proficiency. Therefore, increasing the amount of practice would be an effective strategy for improving the accuracy of range of motion measurements.