A model system was used which enabled the same root canal system to be measured before and after coronal flaring of 51 extracted mandibular molars. The concavity of the distal surface of the mesial root was measured and the amount of reduction was compared after coronal flaring using step-back flared preparation, Gates-Glidden dirll or ultrasonic system(Quick-$varepsilon$) at the furcation and apical 3mm from the furcation. The results were as follows: 1. The mean concavity of mesial root of manchbular molar was $0.73{pm}0.27mm$ at the bifurcation and $0.65{pm}0.23mm$ at the 3.0mm apical from the bifurcation. 2. The thickness of the root canal wall of the mesiobuccal canal was $1.08{pm}0.26mm$ at the bifurcation and $1.00{pm}0.23mm$ at the 3.0mm apical from the bifurcation. 3. The thickness of the root canal wall of the mesiolingual was $1.09{pm}0.21mm$ at the bifurcation and $0.98{pm}0.29mm$ at the 3.0mm apical from the bifurcation. 4. In the amount of reduction at the furcation and at the 3.0mm apical from the furcation there was no statistically significant difference between the step-back preparation and Gates-Glidden drill preparation, and ultrasonic preparation(P>0.05).