The effect of particles on lubricated friction in mixed to hydrodynamic lubrication regime is investigated by using the spherical alumina particles which size distribution is known. It is found that the particles increase friction and the effect disappeared at elevated velocities. The increase in friction is not linearly proportional to the particle concentration but tend to saturate at higher concentrations especially at lower velocities. To explain these major features of the particle effect, a model that the particles larger than the lubricating film interfere with the surfaces is employed. A linear relationship between the numbers of interfering particles and increase in friction is established by introducing a concept that is similar as dead time in pulse counting. Then interference condition is evaluated by means of the interference height, length and friction force generated by each particle. It is concluded that the most of the velocity dependence of the particle effect is caused by the reduction of the interference time and reduction in loads supported by the particles at elevated velocities.Presented as a Society of Tribologists and Lubrication Engineers Paper at the STLE/ASME Tribology Conference in Orlando, Florida, October 11–13, 1999