Abstract: Pin-on-disk friction and wear tests were conducted on N80 oil casing steel disc sample and G105 drill pipe steel pin sample in water-based drilling fluids with different chloride ion concentrations (0, 0.34, 0.68, 1.02 mol·L^-1). The effect of chloride ion concentration on friction and wear properties of N80 steel were investigated. The results show that with increasing chloride ion concentration, the wear rate of N80 steel and the friction coefficient of pin-disc friction pair increased first and then decreased, and both reached the highest values with chloride ion concentration of 0.68 mol·L^-1, which were 2.2×10^-6 mm³·N^-1·m^-1 and 0.85, respectively. During friction and wear in the drilling fluid without chloride ions, abrasive wear mainly occurred in N80 steel; with chloride ions, corrosion-abrasive wear occurred. The chloride ions could destroy the oxidation layer on surface of N80 steel, improving the surface roughness, thereby increasing the wear rate. When the concentration of chloride ions was as high as 1.02 mol·L^-1, the oxidation layer was completely destroyed, making the surface roughness decrease, thereby reducing the wear rate.