A new magnetophoresis method to determine the magnetic susceptibility of single nano/microparticles was developed by applying Brownian motion analysis to determine the size of the particle. This method could measure simultaneously both the magnetophoretic velocity and the radius of the identical single nano/microparticles, which are necessary for the determination of the magnetic susceptibility of the particle. The advantage of this method was demonstrated by the measurement of the diamagnetic susceptibilities of polystyrene particles 500 nm-3 μm in diameter in a paramagnetic 0.5 M manganese(II) chloride solution under a high magnetic field gradient of 5180 T(2) m(-1) generated by a small magnetic circuit.