In this study, the influence of Al2O3 particle amounts on the mechanical, tribological, and corrosion properties of the composite NiP-Al2O3 coating was evaluated. AISI 4140 steel was coated with NiP through an autocatalytic bath with the addition of Al2O3 particles maintained in suspension by mechanical stirring. Following, the coated samples were annealed at 600 °C to increase the hardness and to create an interdiffusion layer, which improves coating adhesion and corrosion resistance. The coating surface was characterized by scanning electron microscope (SEM)/energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), microhardness, wear resistance, and corrosion tests. The results showed that the coating particles’ amount depends on the bath agitation speed, the sample orientation during the deposition, and the volume of Al2O3 particles in the bath composition. Also, the number of particles in the coating affects the deposition kinetics and the thickness of the interdiffusion layer, which affects the wear and corrosion resistance.