Dump truck is one of the vehicles that is often used to transport materials in an infrastructure. Failures often occur in dump truck components, one of which is the pinion gear component. Pinion gear is one of the components that functions to continue the engine rotation to the wheel shaft. In this study, an analysis of the causes of pinion gear failure was carried out on a dump truck with an engine capacity of 3,908 cc. The analysis is carried out to determine the causal factors in the failure. To find the cause of failure, the method uses several tests including, chemical composition testing, macroscopic and microscopic metallographic testing, vikers hardness testing, and fragtographic testing with Scanning Electron Microscope (SEM). Testing of chemical composition obtained non-standard results, where Ni and Mo elements have lower levels, while Cr elements are higher which can lead to increased hardenability. Martensitic phase is visible on the surface and center regions of failed components. The hardness of the material on the surface is higher than in this part which indicates that the material has received case hardening treatment. After analysis in the test, factors that cause failure in the pinion gear are found, including non-standard material composition and improper case hardening treatment. The failure mechanism begins with the occurrence of an initial crack in the pinion gear gear which gets a repeating load and then compresses to the core causing final failure in the opposite part to form fatigue failure.