The purpose of this study was to evaluate the root canal transportation and remaining dentin / cementum thickness after using hand and ultrasonic instrumentation in the curved mesial root canals of extracted human mandibular molars. Fourty - six clear polyether blocks were made and randomly divided into two groups: hand instrumentation group with K - Flex files and ultrasonic instrumentation group with Suprasson SP unit. All root canals were instrumented to a size corresponding to a # 30 K - Flex file 1mm short from the radiographic apex. The roots were then sectioned perpendicular to the long axis so the apical and middle third could be evaluated with the Zoom stereomicroscope. The results were as follows : 1. In the total amount of removed dentin at middle third level, there was not significant difference between the hand instrumentation and ultrasonic instrumentation (P>0.05). 2. In the total amount of removed dentin at apical third level, there was more removed by the ultrasonic instrumentation than hand instrumentation(P<0.005). 3. In the transportation width, there was not significant difference between the two groups at both sectioned levels(P>0.05). 4. It was suggested that the canal was transported distally at middle third level and mesially at apical third level by booth techniques.