This study was done to evaluate transportation of the apical foramen after 0.5 ㎜ overinstrumentation by ProFile, ProTaper and K 3 in simulated resin root canal. Sixty simulated resin root canal with a curvature of J and S-shape were divided into two groups. Each group consisted of three subgroups with 10 blocks according to the instruments used: ProFile . Simulated resin root canal was prepared by ProFile, ProTaper and K 3 with 300 rpm by the crown-down preparation technique. Pre-and post-instrumentation apical foramen images were overlapped and recorded with Image-analyzing microscope 100X (Camcope, Sometech Inc, Korea). The amounts of difference in width and dimension on overlapped images were measured after reference points were determined by Image Analysis program (Image-Pro � Express, Media Cybernetic, USA). Data were analyzed using Kruskal-Wallis and Mann-Whitney U-test.The results suggest that ProFile showed significantly less canal transportation and maintained original apical foramen shape better than K 3 and ProTaper. [J Kor Acad Cons Dent 32(2):87-94, 2007]