In a method of producing magnetic particles which includes the steps of preparing alloy particles capable of forming a CuAu or Cu.sub.3Au hard magnetic ordered alloy phase and of forming magnetic particles for forming CuAu or Cu.sub.3Au magnetic particles, a plurality of solutions L1 and L2 for preparing the alloy particles are passed in a thin-plate laminar flow and diffused in the direction perpendicular to the flow direction at the contact interface of the solutions L1 and L2 in a mixing channel by using a microreactor, whereby a uniform mixing reaction is conducted in a short time.