Aim: to evaluate the maximum torque required for of fracture of the instruments of the rotary systems: Profile, K3; and Protaper, by application of torsion test. Methods: 300 NiTi rotary instruments were divided into 5 groups according to the brand and taper. All instruments were submitted to the torsion test by means of a device attached to a mechanical testing machine, for achievement of the maximum load values applied to each instrument. Such values were converted into the maximum torque to fracture and statistically analyzed by the Student's t test. Results: the study demonstrated a significant difference between instruments with tapers 0.04 and 0.06. There was also difference between the Profile and K3 systems for taper 0.06 and regardless of the taper (p=0.000). However, the Profile and K3 systems presented similar performance for taper 0.04 (p=0.119). The Protaper instruments were similar to the Profile instruments 0.04 (p=0.119), Profile instruments 0.06 (p=0.178) and K3 instruments 0.04 (p=0.977). However, the instruments of the Protaper system displayed lower fracture strength than the K3 instruments 0.06 (p=0.000). Conclusions: the K3 instruments 0.06 were more resistant to fracture than the Protaper instruments and were significantly more resistance to torsional fracture than those of the Profile system. However, the results demonstrated no significant difference between the Profile system 0.04, Profile system 0.06 and K3 system 0.04 when compared to the Protaper system, as well as comparison between the Profile system 0.04 and the K3 system 0.04. However, the K3 0.06 instruments were more resistant to fracture than the Profile 0.06 instruments.