The present study aimed to evaluate the ability of DOM to demonstrate the signs of instrument's distortion and to investigate types and frequency of rotary NiTi twisted files (TF) deterioration modes. Methodology: Eighty-eight discarded TF instruments after clinical use were experimented with. They were grouped into 4 groups of 22 file each according to the TF instrument taper from taper 0.04, 0.06, 0.08, and 0.010. Files were cleaned and sterilized before examination of surface deterioration signs and/or tip fracture. Two evaluators were separately examining the files and Kappa analysis was done. Results were statistically analyzed (significance level 0.05). Four types of deterioration were depicted. Unwinding of the instruments at the third-fourth flute was the mostly found deterioration type for all tested tapers (33%). It occurred most frequently in 0.08, 0.06, taper. This was followed by instruments' tip separation (15%) with the maximum number was found in taper 0.08. no one of the examined tapers 0.04 showed tip separation. However, it showed the highest number of tip turn-over among the tested TF instruments. Over-twisting was only found in 5% of all tested TF files. Conclusions: 40% of TF files discarded after clinical use and examined under SOM showed various signs of deterioration mostly blade unwinding and tip fracture .DOM images were clear and focused, allowed easy and on spot immediate evaluation of signs of deterioration