Magnesium ion-exchanged a-zirconium phosphates (Mg-a-ZrP) with particle sizes of 600 and 80 nm were prepared through the sealed ion-exchange and one-step hydrothermal synthesis methods,respectively.It was found that larger particles of Mg-a-ZrP had a higher load-carrying capacity than that of smaller particles,whereas smaller Mg-a-ZrP particles had better anti-wear properties than that of larger Mg-aZrP particles under mild loads.The correlation between the particle size of the sample and the surface roughness of the friction pair thus seems to be a key factor influencing the performance.