Aim: This study was conducted to evaluate the effect of preflaring and file size on the accuracy of the Root ZX and E-PEX Pro electronic apex locators (EALs). Materials and Methods: The actual working length was set 1 mm short of the apical foramen in the forty extracted mandibular premolars. The teeth were embedded in an alginate mold, and two examiners performed the electronic measurements using #10, #15, and #20 K-files. The files were inserted into the root canals until the “0.0” or “APEX” signals were observed on the light-emitting diode or display screens for the E-PEX Pro and Root ZX, respectively, retracting to the 1.0 mark. The measurements were repeated after the preflaring using the S1 and SX Pro-Taper instruments. The mean differences between the actual length and electronic length values were analyzed by the Wilcoxon signed-ranked test with a significance level of P < 0.05. The factors evaluated were “Accuracy of EALs,” “size of file,” and “presence of preflaring. Results: No significant differences were observed in the accuracy of both EALs when compared with the actual lengths. On the size of the file, significant difference noticed with #20 K-file. The preflaring procedure significantly ( P < 0.05) increased the accuracy of the measurements for the Root ZX and Epex Pro. Conclusions: The tested EALs showed acceptable accuracy, whereas the preflaring procedure revealed a more significant effect than the used file size.