Tribological properties of neat 150 SN mineral base oil and the oils doped with different contents of tricresyl phosphate (TCP) under magnetic field or non-magnetic field were evaluated on a four-ball tribotester, on which an extal magnetic field was applied. Furthermore, the morphology and the tribochemical characteristics of the w surfaces were examined with a scanning electron microscope (SEM) and an X-ray photoelectron spectrograph (XPS). The tribological test results showed that the magnetic field improved anti-wear properties but impaired the friction-reducing properties of neat base oil and the TCP-doped oils. The w surfaces tested in magnetic field were characterized by a slighter wear than those tested in normal condition, especially for the surfaces lubricated with the TCP-doped oils. Furthermore, the results of XPS analysis indicated that tribochemical films on the surfaces tested with TCP-doped oils were mainly composed of ferric-containing compounds such as Fe2O3, Fe3O4 and FePO4. Under the influence of a magnetic field, the atomic concentrations of oxygen and phosphorous in the frictional sub-surfaces were higher than those without magnetic impact. Thus it can be in-ferred that the improved anti-wear properties and impaired friction-reducing capabilities of lubricating oils with TCP under a magnetic field were related to the diffusion of phosphorus and oxygen into the substrate induced by magnetism.