Abstract: Objective　This research aimed to study and assess clinical application of the using of three Nickel-titanium (NiTi) rotary instruments, namely ProFile, ProTaper and Hero 642, in preparation of curved root canals.Methods　　80 teeth with curved root canals were instrumented by ProFile, ProTaper or Hero 642 rotary instruments using crown-down technique in test groups and by stainless steel K files in control group. All teethwere obturated with lateral condensation method. The efficiency of preparation and obturation was analyzed with radiographs before, during and after operation.Results　Three NiTi rotary instru- ments could keep the curvature and flow of curved canals very well, and the continuously tapered root canal shape was achieved. There were no transportation, apical blockage, and ledge in all test groups. The operative time was shorter for ProTaper than that of ProFile and Hero 642. Endodontic flare-up seldom occurred in test groups. Two rotary instruments fractured in canals.Conclu- sion　With the NiTi rotary instruments, ProFile, ProTaper and Hero 642, curved root canals can be prepared effectively and quickly. Meanwhile, the problem of instrument separation should be prevented.

Key words: curved root canals, nickel-titanium instruments, root canal preparation, instrument fracture