The purity of industrial liquids and gases is effected by particles having micron and submicron sizes. Many of the particles in those streams are ferro- or paramagnetic substances. Because of the low concentration of these particles, classical filters (mechanical, separator, etc.) do not perform significant filtration process, and magnetic filters (MFs) which have magnetized packed bed show greater capacity of capturing these particles. Also, MFs show different properties of non-stationary performance characteristics compared to the other type filters. The variation of the MF performance with time is initially constant for some period of the filtration process but falls after that filtration time. A magnetic filtration equation which takes these properties into account has been developed. This equation includes magnetic, hydrodynamic and geometric parameters of the filtration system. The equation obtained is a linear partial-differential equation, which must be solved analytically. The theoretical results obtained from the analytical solution have been compared with the experimental results obtained from various industrial areas using MFs. It can be concluded that a good agreement was found.