Surface modifications of nickel-titanium instruments such as electropolishing may reduce fracture incidence but could potentially alter the instruments' mechanical properties. We studied the impact of two surface types, machined and electropolished, of ProTaper shaping files on torque and force during simulated root canal preparation. Pilot holes of 0.5-mm diameter were drilled perpendicularly through 3-mm thick human dentin discs and served as standardized simulated root canals (SSRCs). A total of 300 SSRCs were prepared with ProTaper Sx, S1, and S2 instruments using an automated testing platform. Maximum torque (in Ncm) and maximum force values (in N) were measured. In three experimental groups (Sx, S1, and S2) preparation of SSRCs was performed using electropolished versus machined instruments in the same dentin disc. For all three tested instruments, peak torque was higher for electropolished files; apically exerted forces were similar, except for Sx, in which machined instruments developed higher forces. We conclude that surface treatment by electropolishing alters the cutting ability of nickel-titanium rotary instruments.