In this research, slurry erosion-corrosion and slurry-erosion resistances of electroless Ni-P coatings have been investigated. These coatings were applied on AISI 5117 steel discs by electroless deposition process and then they were heat treated at 650 C for 1 h. Slurry erosion-corrosion and slurry-erosion resistances of deposits were measured by the whirling-arm tester. Also, microstructure of deposits before and after heat treatment was evaluated by X-ray diffraction (XRD) analysis. The results showed that the electroless Ni-P plating greatly increased the erosion and erosion-corrosion resistances for all impact angles. Moreover, the effectiveness of plating was the highest for an impact angle of 45o , where the erosion and erosion-corrosion resistances were increased by 60% and 70% respectively, compared with that of the substrate. The results showed also that the coated and blank specimens behaved as ductile materials under erosion and erosioncorrosion tests and the maximum weight loss occurred at an impact angle of 45o .