The purpose of this study was to determine the accuracys grade of the mini apex locator in the location of the CDC when this it indicates the measure of 0.0mm, confirmed with the SEM. 84 root canals were taken from permanent extracted lower first molars for periodontics of prostodontics reasons this were separated of the distal roots, cut them to level of the enamel-cementum joint, with a carbide disk to low speed, to later on the COE was made using gates glideen burs # 2y 3, irrigating each one of them with NaOCl to 5.25% and later they were introduced sybronendo K flex files #0.8, 10, or 15 depending of the root canal diameter. The Asakos method was used for determine the work's length, followed step was to fit the files inside of the root canal with fluid resin, subsequently to proceeded to cut the filess handle with a and to observe the position of the D0 of the file with respect to the CDC with the SEM with a magnification of 400X. The results show that in the 85.7 %( 52 roots canals) of the samples the D0 of the file was between 0.0 -0.5mm of the CDC and the 14.3 %( 12 root canals) was for under of the CDC. Test showed no significant differences between measurements demonstrated like that the accuracy of Mini Apex locator in determining the working length of root canals during endodontic treatment.