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Purpose: To investigate, in vitro, the magnitudes of vertical load and torque generated during post space preparation of root canal treated teeth and their influence on microcrack development. Materials and Methods: Forty extracted premolars with single canals were decoronated. The roots were prepared using ProTaper Next system (Dentsply Sirona) to X3 file, obturated, and provisionalized. After a 3‐week incubation period, the roots were distributed into 2 groups according to the post drill system used (ParaPost fiber post and RelyX fiber post). During post space preparation, the vertical load, torque, and preparation time were recorded. Before and after the post space preparation, the roots were imaged with micro‐computed tomography (SkyScan 1172; Bruker micro‐CT) to detect new microcracks. The data were analyzed using the Mann‐Whitney U‐test and chi‐square test. Results: In ParaPost and RelyX, the post space was prepared in 48.31 ±25.28 and 47.71 ±13.36 seconds, respectively (p = 0.360). The peak vertical load generated with ParaPost (20.23 ±7.80 N) was significantly lower than that of RelyX (29.43 ±5.82 N) (p = 0.010). The maximum torque attained with ParaPost (1.42 ±0.61 N.cm) was found to be significantly lower than that of RelyX (3.23 ±1.58 N.cm) (p = 0.000). No post drill fracture or visible deformation was noticed throughout the experiment. New microcracks were found in three ParaPost samples only. Conclusions: The choice of postdrill influenced the loads and torques generated during postspace preparation. The ParaPost system showed favorably lower load and torque values during the postspace preparation compared with the RelyX system. [ABSTRACT FROM AUTHOR]