PURPOSE: The purpose of this study was to compare the conventional and digital radiography in the estimation of working length in mandibular molars. MATERIALS AND METHODS: Sixty molar teeth were selected and divided into three groups in the basis of canal curves (0-15degrees, 15-30degrees, >30degrees). After the placement of a 15 K-file, radiographs were taken with a conventional film (F-speed) and a digital sensor. Canal lengths were measured in these images by two observers. Statistical analysis was performed with repeated measures of ANOVA and paired sample t-test with 95% confidence. RESULTS: There was a high inter-observer agreement on the measurements of working length in conventional and digital radiographs. There was no significant difference between the mean values of measurements in conventional and digital radiography. Moreover, there was no significant difference between conventional and digital radiography with the actual values in the basis of canal curves. CONCLUSION: The accuracy of conventional and digital radiography in the determination of the working length was in an acceptable range.