Background : The cun measurement system, an essential and convenient method in locating acupoints, has been widely used in the practice of acupuncture. However, traditional cun measurement has been criticized for its lack of reliability. Objectives : The purposes of this study are to determine if one cun measured by the directional methods have a consistency with that of proportional methods and to investigate which factors are related with these differences, especially in forearm. Methods : The distance between the elbow crease and the wrist crease of forearm was compared to a reference value of one cun obtained by the directional method. In this method, one cun is one third of the distance between index finger and small finger of a subject, measured at proximal interphalangeal joint. In addition, to investigate the factors influencing the differences between these two methods, we measured the height and body weight and calculated body mass index (BMI). Finally we analyzed the factors correlated with these lengths by linear regression test. Results : The results showed that one cun obtained by the directional methods were significantly different from one cun by the proportional methods in forearm. It was demonstrated that the length acquired with the directional method was more correlated with body weight and body mass index, while the length obtained by the proportional method was more correlated with the height. Conclusion : These findings suggest that the directional method is less likely dependable in locating acupoints than the proportional method because the influencing factors are different.