Aims: The aim of this study is to compare three different methods of measuring root canal curvature and its correlation with canal access angle (CAA) in curved mesiobuccal canals of permanent mandibular first molars. Materials and Methods: Sixty human mandibular first molars were used in this study. Standardized access cavities were prepared. After endodontic access, a size 10 K-file was placed in the mesiobuccal canal extending to the apical foramen, and radiographs were taken. Radiographs of each root canal were taken in buccolingual direction with the long axis of the root perpendicular to the central X-ray beam. After that, the radiographs were scanned with a computer Scanner. The angles were measured using the Schneider method, Weine's method, Lutein's method, and correlated with the CAA method. The angular and linear values used in this study were plotted, and the pertinent measurements were made using the program AutoCAD R12. Statistical Analysis: The resultant values were evaluated statistically using ANOVA test and Pearson correlation and multiple regression analyses ( P = 0.001). Results: Lutein's method is as effective as the Schneider's method, Wein's method and CAA. method in evaluating root canal curvature. Conclusions: Scheider/Wein/Luiten method, together with CAA method, may be considered as a reliable guideline for preoperative assessment of canal curvature.