Abstract: Longitudinal cracking took place in threaded section of some micro-alloyed steel hex during the quenching ＆tempering heat treatment. The root cause of crack formation was investigated using optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), Vickers hardness testing and spectroscopy. Experimental results indicated that the chemical composition, microstructure and hardness of the bolts meet all the specification requirements; and open-type folding defects with certain depth present in bolt surface is the root cause of cracking in heat treatment. The open-type folding defects in cracked bolts exhibited obvious decarburization and oxidation which resulted in honeycomb structure composed of fully decarburized ferrite and surrounding oxide films. Oxide particles (Fe,Mn,Si)O formed by internal oxidation distribute in the local matrix near the folds. During cooling of quenching process, significant stress concentration addressed at tips of these folds, which lead to the initiation of cracks under the combined effect of thermal stress and phase transformation stress. These cracks propagated approximately 7 mm in depth radially toward the core. In contrast, no folding defect was observed on the surface of uncracked bolts. The free surfaces of quenched cracks undergo oxidation during medium-temperature tempering process, forming a dense film of ferrous oxide in submicron-scale, without any decarburization.