Objective: To investigate the effect of the hand file size on the accuracy of Root ZX (J. Morita Co., Kyoto, Japan) and iPex (NSK, Tochigi, Japan).Methods: Seventy-five single-rooted teeth were decoronated, and canals were coronally flared with Gates Glidden burs sizes 4, 3, and 2. Actual canal length was determined by averaging two readings obtained by inserting K-file size 8 until its tip was apparent at the most coronal border of the apical foramen. The reference length was actual length-0.5 mm. The blinded operator utilized Root ZX and iPex following the manufacturer’s recommendations. Teeth were placed in sponge blocks soaked with Ringer’s solution. Canals were irrigated with 5% sodium hypochlorite. K-file size 8 was attached to the lip clip and introduced until the APEX/0.0 mark, then withdrawn to the 0.5 mark. A stable meter gauge for five seconds indicated an acceptable reading. Readings with sizes 10 and 15 were obtained afterward. All measurements were done twice, then averaged. Data analysis was done using ANOVA and a posthoc Bonferroni test with the significance level set atP<0.05.Results: For Root ZX, the mean length with size 8 was not significantly different from the mean reference length (P=0.205). The same was found for its mean length at size 10 (P=0.093). However, the mean Root ZX length with size 15 was significantly shorter than the mean reference length (P=0.019). Mean iPex lengths with sizes 8, 10, and 15 were all significantly shorter than the mean reference length (P=0.038, 0.006, and 0.02, respectively).Conclusion: The size of the hand file affected the precision of Root ZX and iPex.