Abstract: The orthogonal array tests were carried out to investigate the effects of velocity, load and pin surface roughness on the tribological properties of steel and nylon PA66 disc friction pairs under dry sliding friction condition on the MMW-1A vertical universal friction wear test machine. According to the range analysis and variance analysis, results show that load and pin surface roughness have significant influence on the friction coefficient and wear mass while the effect of speed is less significant. And the best combination of parameters for 45 steel/PA66 friction pairs are 50 N of load, 11.25 m/s of velocity, and R_a 0.6 μm of the pin surface roughness. Based on the optimum condition, the control variable experiments were carried out. Under the load lower than 90 N, the wear of PA66 are mainly adhesive wear. Under the load of 90 N, the wear of PA66 is the abrasion wear and adhesive wear. Under the load of 140 N, the wear of PA66 is the adhesive wear accompanied by seizure. When the pin surface roughness (R_a) is lower than 0.46 μm, the main failure form of PA66 is adhesive wear. When the pin surface roughness (R_a) is 0.46 μm, the main failure is due to abrasive wear and adhesive wear, and a continuous transfer film is found on the surface of the metal pin. When the pin surface roughness (R_a) is larger than 0.46 μm, the main failure is due to the ploughing effect.