The objective of this study was to evaluate the fracture risk of ProTaper nickel-titanium (NiTi) rotary instruments under a continuous versus an alternating rotation when using the same instrumentation sequence. Plastic blocks with an embedded artificial root canal were used to investigate the fracture risk of NiTi instruments under a continuous rotation (360 degrees) and an alternating rotation (clockwise 60 degrees and counter-clockwise 45 degrees). A new instrument set was used until fracture of any instrument occurred. The number of times an instrument set and instrument type were used before fracture occurred was recorded and statistically evaluated. The results obtained in this investigation showed that the fracture risk of ProTaper instruments when used under continuous rotation was significantly higher (52.14% / P0.001) compared with use under an alternating rotation. Keywords: alternating rotation, continuous rotation, instrument separation, NiTi rotary instruments