Background and Purpose: Normal functions of the foot depend on the mobility and stability of the first ray; but, there is lack of a valid and reliable tool to measure its motion in clinical settings. Previous literatures have implicated that the first ray ruler, the only quantitative tool in clinical setting, had a poor intra- and inter-rater reliability. In a recent study, the use of a modified first ray ruler and standardization of the testing position improved the reliability to an excellent level. However, its validity has not been established yet. The purpose of this study, therefore, was to determine the validity of measuring first ray mobility using a modified first ray ruler as compared to the results from X-ray imaging. Methods: Five young healthy male adults participated in this study. Five trials of the first ray mobility test were performed and dorsal and plantar mobility of the first ray was measured using a modified first ray ruler. The fifth and the averaged data were used for analyses. X-ray images were taken at the end of the 5(subscript th) trial as the gold standard for first ray mobility variables, which were measured 3 times by the same rater using the Picture Archiving and Communication System. The averaged data from the X-ray measures were used for analyses. Pearson correlation coefficient analysis was performed for two measurements to determine the validity. Results: The results suggest that measurements of first ray mobility using the modified first ray ruler arc highly correlated with the gold standard X-ray method (r greater than 0.85). indicating good validity of measures. Conclusion: Since the modified first ray ruler has the advantages of high-validity. low-tech-design, easy-application, inexpensive, portable, and reading data immediately, this study provides support for the use of this simple-designed ruler as a quantitative measure for the purpose of evaluating first ray problems in clinical or laboratory settings.