The rear axle shaft is a component of the dump truck that functioned to forward the engine's rotation to each tire. The rear axle shaft is a shaft that is connected to the axle directly and serves as a power or rotation forwarder. This study conducted an investigation of the reasons for failure in the rear axle shaft dump truck of 7,5 Ton, with the goal of determining the case and process of the happened of failure. The study is to examine the causes and mechanisms of failure in the rear axle shaft with Fractrography testing, both macroscopically or microscopically, hardness testing and tensile testing. Then, the mechanical characteristics test revealed yield strengths of 655 MPa, and tensile strengths of 795 MPa. The value is beyond the standard of AISI 4140, that is yield strength 690 MPa and tensile strength 810 MPa. The layer thickness is 9 mm (the standard is 2-3 mm) and the hardness value obtained by 48 HRC that is the upper of the standard case hardening material, the composition test results are as the standard. The hardness value obtained is not equally distributed around the rear axle shaft, and the composition test results are within the acceptable range of standard. After the research was done, it clarified that the failure of the rear axle shaft dump truck at 7.5 tons was caused by a heat treatment error on the rear axle shaft. An initial crack at the axle shaft's edge initiates the failure mechanism, which then propagates to the core area owing to repeated loads and strains, leading to a final fracture at the rear axle shaft's core, becoming a fatigue failure.