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Purpose: This paper aims to investigate the effect of fatigue behaviour on fracture in the 4th gear (helical gear) of a pick-up truck. Design/methodology/approach: Fracture on the failed helical gear is characterised through metallographic and fractographic analyses. Mechanical testing and finite element simulation are employed to assess the factors contributing to the gear failure. Findings: The microstructure observed in the case layer was martensite, leading to a hard and brittle surface due to carburising. Failure initiated at the crack origins and then propagated to the instant fracture zone in the core of the gear tooth. Multiple crack origins accelerated the development of ratchet marks, attributed to the high intensity of stress exerted on the workpiece and ultimately leading to a substantial final overload zone. Hardness decreased with increasing depth of the gear surface due to the effects of carburising and hardening treatments. Stress was initiated from the contact stress on the gear tooth surface and transformed into bending stress along the central axis of the gear. The contact stresses became critical when the torque surpassed the contact strength of the material. Research limitations/implications: Simulation samples must be experimentally validated to improve the results. Practical implications: Metallographic and fractographic analyses are crucial in elucidating the wear mechanisms in mechanical components. Additionally, finite element analysis can indicate the influence of stress on the mechanical part, providing insights that can effectively guide the limiting transmission power to ensure extended service life. Originality/value: Cost reduction, time for analysis, and finding the root causes of the problem should be conducted to improve the implementation process, leading to high product quality. [ABSTRACT FROM AUTHOR]