We developed a method of on-line measurement of the magnetic moment of aerosol particles. For this purpose we determine the deposition of the particles in a filter consisting of magnetic nickel screens. Deposition is governed by the dipole force magnetic particles experience near the magnetized wires of the screens. The number concentration at the in-and outlet of the filter is measured by optical particle counters. The so called single fiber efficiency is derived directly from the measured data as a function of particle size, particle magnetic moment and fiber magnetization. The relationship between this dimensionless value and the magnetic moment is obtained by a computer simulation using trajectory analysis. The method is tested with equally sized iron/iron oxide particles in the size range from 40 to 170 nm.