Phosphorous (P) ion implantation was performed on a steel substrate to investigate the effect of the P implantation on the friction characteristics of steel lubricated with oil. To determine the characteristics of the P-implanted layer, several kinds of surface analyses were carried out, resulting in a correlation between the X-ray photoelectron spectroscopy depth profile and the thickness from spectroscopic ellipsometer. A three balls-on-disc tribometer was employed with sliding speeds ranging from 3 mm/s to 40 cm/s. The friction coefficient of the P-implanted steel was relatively lower at speeds below 10−1 m/s, compared with the unimplanted steel. This could account for both the sacrificing nature and the adhesion behaviour of the P-implanted layer.