Abstract: In the lubricated systems of some machines, e.g.mining machines, many contaminant particles are in suspension in the oil. These particles are generally too small to be removed by the filter and remain in suspension in the circulating oil. When the wear condition of the machines is monitored by ferrograph, many contaminant particles are deposited onto the substrates and cover the metal wear particles. This is a major problem affecting ferrogram examination and wear monitoring. So far, in order to solve the problem, two methods are developed: 1） The dilution of the examined oil samples is increased;2）The ferrograms are prepared using rotary particle depositors. There are two defects happened: increasing the dilution reduces the number of wear particles included in the samples， and ferrograms prepared by rotary particle depositors are not convenient for the quantitative analyzing of the density and size distribution of wear particles. In this paper the effect of magnetic force and gravitation on various particles was studied. The separating process of wear particles in machine lubrication oil by highgradient magnet fields was analyzed． The magnet force, gravitation and damping force acting on the particles were investigated． A mathematics model that can describe separating deposition of the particles was developed. Experimental work shows that the model correctly expresses the particle depositing process. Changes of magnetfield parameters were studied based on the mathematics model．The results show that adjusting magnet field parameters can control the particle depositions and improve the ferrogram examination and wear monitoring．