A comparative study of two designed root canal enlarging instruments on the preparation of curved root canal. Aim to compare two rotary Ni-Ti systems NRT and Flexmaster rotary systems regarding changes in canal curvature, remaining wall thickness and presence or absence of smear layer. Methodology root canals of extracted molars were shaped with Flexmaster and NRT instrumented in resin blocks, the apical preparation size was 35. A re-assembly technique allowed the comparison of the canals before and after preparation using direct digital radiography for canal curvature and stereomicroscope for canal wall thickness and smear layer. Results the changes in the canal curvature was found that Fexmaster group recorded statistically significant higher changes in canal curvature postoperatively (3.219 ± 0.4814%) than NRT group (1.681 ±0.2478 %) as indicated by t-test (p≤0.05). in the canal wall thickness Flexmaster group recorded statistically significant higher changes in canal wall thickness postoperatively than NRT group as indicated by t-test (p≤0.05).in the canal debris the use of Naocl and EDTA with Flexmaster and NRT groups recorded significantly lower canal debris than Naocl only. Conclusion the evolvement of mechanized instrumentation using specially designed NiTi files has revolutionized endodontics to be elevated to a higher level due to efficiency maintaining canal curvature, remaining wall thickness and reduction in the smear layer.