The purpose of theis study was to evaluate the effectiveness of ultrasonic instrumentation in removing debris and smear layer from the root canal walls. 54 extracted, permanent single rooted teeth were randomly divided into 6 groups of 9 teeth. After canal preparation, the teeth were longitudinally sectioned. The sepcimens were then naturally dried for 2 days, given a maximun thickness gold coating, and examined under the SEM (JSM-35C type, JAPAN). Photographs of all specimens were then taken of the middle and the apical third of the root canal wall. The results were as follows: 1. In all groups, debris and the smear layer were not completely removed from the canal wall 2. There were no significant differences between at the apical third and at the middle third in removing debris and the smear layer in all groups. 3. There were no significant difference between the step - back group and the ultrasound group in removing debris and smear layer. 4. In general, the step - back/ultrasonund groups showed greater canal debridement than the step - back group or ultrasound group. 5. The step - back/ultrasound group with a No. 25 file for 3 min. showed significantly greater canal debridement than the step-back group (p<0.05), or the other step - back/ultrasound groups(p<0.05).