Aim: The present in-vitro study was undertaken to evaluate and compare fracture resistance of teeth ,following canal preparation by hand instrument(k file) and rotary instrumentations (pro taper, Hero s file). Methodology: Forty-five mandibular premolars teeth were divided into three groups of 15 teeth each. The Control group is non instrumented teeth. Group I, canals were prepared with step-back technique using standardized stainless still K-files. Groups II and III, canals were prepared with crown-down technique using hero s, ProTaper rotary files respectively. All root obturated with gutta-percha and AH26 sealer. The roots were vertically loaded by using a universal testing machine. The results were statistically analyzed using ANOVA and Mann-Whitney U-test. Result: The Mean of fracture load was 520 Newton/mm2for K-files, 345 Newton/mm2 for pro taper files, 321 Newton/mm2for Hero s files , the differences between rotary and hand groups were statistically significant (p<0.05)but not significant between two groups of rotary instruments (p>0.05) .vital teeth has higher resistance to fracture than endodontic treated teeth and also result represent that resistance to fracture higher in hand instruments than rotary instruments.Conclusion: Amount of cutting of dentin during instrumentation affects strength of teeth .so after endodontic treatment the tooth must be properly filled to prevent load on gutapercha and caused vertical root fracture ,Protaper and heros rotary files reduce resistance of tooth significantly because of different taperity and reduction ,so to compensate the weakness due to instrumentation we can use new system of obturation like system B instead of lateral condensation.