The purpose of this study was to compare the cutting efficiency of the following rotary nickel-titanium instruments: Alpha-File (Komet, Lemgo, Germany), FlexMaster (VDW, Munich, Germany), Mtwo (VDW, Munich, Germany), ProFile (Dentsply Maillefer, Ballaigues, Switzerland), and RaCe (FKG, La-Chaux-de-Fonds, Switzerland). New .06/25 and .04/35 instruments were used; sample size was 12 for each group. Cutting efficiency of all 120 instruments was determined in a rotary working motion by means of a computer-driven testing device. Special plastic samples with a cylindrical canal were used, and the maximum penetration depth of the instruments into the lumen was the criterion for cutting efficiency. Statistical analysis was performed by using analysis of variance and post hoc Student-Newman-Keuls test. Out of all .06/25 instruments, Mtwo and RaCe displayed significantly greater penetration depths than all other instruments, whereas for the other size FlexMaster, Mtwo, and RaCe were significantly superior to all other instruments (p < 0.05). For both sizes, the significantly lowest values were obtained by ProFile (p < 0.05). Under the condition of this study, Mtwo and RaCe displayed the greatest cutting efficiency.