Background: Instrumentation of root canals significantly weakens the roots. Root stresses generated from inside the root canal are higher in the apical region. Aims: To compare the effect of different root canal preparation techniques & instruments on the development of apical root cracks. Materials and methods: Forty extracted mandibular premolars were mounted in a resin block with simulated periodontal ligaments and divided into four groups of 10 each (Group A, Group B, Group C, Group D) according to root canal preparation technique and instrument. Group A: The teeth were instrumented with stainless steel K-flex files using a step back technique with balanced force concept, Group B and Group C teeth were instrumented with hand protaper and rotary protaper with crown-down technique, Group D: Teeth were instrumented with rotary K3 system with crown-down technique. Result: 3 apical root cracks were found in Group A (K-flex files) amongst 10 and 1 Apical root crack in Group B (hand protaper). No crack was found in samples among group C and group D. Conclusion: Development of cracks on the surface of the apex during mechanical preparation might lead to future fracture of the tooth structure.