The purpose of this study was to evaluate the resulting root canal angulation and shape change after using various enlarging instruments. The mesial canals (120) of extracted human mandibular 1st and 2nd molars were randomly divided into 6 groups; Control, K-type me, Heliapical me, Canal Master me, sonic and ultrasonic instrumentation group. Vertical angulation of each canal was determined by a straight line through the long axis of canal and another straight line through the apical foramen to intersect at the point where the canal began to leave the long axis of the tooth. By recording and comparing the measured angles of the each set of pre-and postinstrumentation. Then, the roots were sectioned horizontally in the apical, middle, coronal thirds and the canal shapes examined, as was the mesiodistal canal diameter as it relates to the external root surface. The results were as follows: 1. Instrumentation using K-type me group resulted in the highest mean change in angulation ($9.900^{circ}$) (p < 0.005), while Sonic Air MM 3000 group resulted in the least degree of straightening canals ($8.250^{circ}$) (p < 0.005). 2. Canal Master file group resulted in the best canal shape at the three levels (P < 0.005). 3. Measured minimal mesial root width produced Heliapical me group at the apical 1/3 level, Sonic Air MM 3000 group at the middle 1/3 level, Heliapical me group at the coronal 1/3 level (P < 0.005). 4. Measured minimal distal root width produced Sonic Air MM 3000 group at the apical 1/3 level (P < 0.05), Heliapical me group at the middle 1/3 level (P < 0.005), Canal Master me group at the coronal 1/3 level (P < 0.005). 5. HeIiapical me group produced more increased canal diameter than any other groups (P < 0.005).