Considering the flow regime of the fluid through the pores of a magnetic filter having a granular matrix composed of ferromagnetic spheres and the accumulation process of the captured particles. the filtration mechanism is investigated. The effect of the different parameters of the filtration system on the efficiency of the magnetic filter is identified. New mathematical expressions are obtained to explain some events which take place in the filtration mechanism and which have not been described completely in the literature so far. The efficiency of the magnetic filter is expressed in terms of the ratios of the magnetic and static pressures which can be measured or identified easily. The theoretical results are compared with the experimental data and it is concluded that they are in good agreement.