The diffusion capture of submicron-sized particles in the gradient field formed by the magnetized ferromagnetic spheres in the external homogeneous magnetic field has been theoretically investigated. Magnetic field measurements were made with a specially designed prototype magnetometer. The simple semi-empirical formula of the magnetic force acting on the submicron particle was determined. The analytical solution of the diffusion equation for steady states was obtained. The concentration distribution of the particles in capturing regions was determined. A formula was obtained to evaluate the critical size of the submicron particles captured in this area. The effects of magnetic, hydrodynamic and geometric parameters of the investigated system on the critical size of the particles in the diffusion event were evaluated. The critical size of the particles decreased with the increase of the external magnetic field and was found to be independent of the dimensions of the spheres. As a result; the diffusion separation phenomenon in the gradient field formed by the magnetized ferromagnetic spheres is more effective than in the gradient magnetic field formed by the wires. The results obtained contribute to the development of the theory of magnetic separators using ferromagnetic matrix elements.