Objective To evaluate shaping ability of nickel- titanium rotary instruments（ProTaper） in curved root canals. Methods The computer assistant measure system of simulated root canal was used for quantitative analysis of root shape of eight simulated root canals during ProTaper shaping process. Blue ink was injected into the simulated root canals before instrumentation, when F2 reached work length（WL）（F2） and when F3 reached WL（F3）. Orifices and apexes were sealed with adhesive tapes. Root canal images were gained by Mustek BP4800TA scanner. The images before and after preparation were turned into Di - numerical images and fused. The changes of central line positions and curvatures were measured in quantity through this system. Data of seven preestablished point from orifice to apex were recorded and analysed by SPSS 10.0 soft ware. Results ProTaper system reduced the angles of simulated canals. Selecting F2 or F3 as the master apical file（MAF） had no influence on the changing rate of the root angles. ProTaper had excess cutting on the outer wall of orifice and the inner wall of curve part, but the displacement of apex was minimal（only 0.021 mm）. Conclusion ProTaper has preferable shaping ability. To prepare very difficult curved canals, the master apical file（MAF） could be F3. Attention should be taken when preparing some curved canals with hollows in their inner walls, in order to avoid the lateral perforation.
