By "flow tubular reactor method", spherical ferrite nanoparticles with 210±30 nm diameter and composition intermediate between Fe₃O₄ and γ-Fe₂O₃ were synthesized from an aqueous solution. A reaction solution of FeCl₂ and an oxidizing solution of NaOH and NaNO₃ added with sucrose, which shapes spheres, together with ferrite seed crystals, which narrow the size distribution, were mixed and injected into a flow tubular reactor kept at 90°C. The tubular reactor had 3 mm inner diameter, 4 mm outer diameter, and 50 m length. We extended the method to a sequential flow process by which we could fabricate ferrite nanoparticles coated with citrate; the output stream of the tubular reactor containing the as-synthesized ferrite particles was mixed with a trisodium citrate solution, which was then injected into another tubular reactor at room temperature. The obtained ferrite particles coated with citrate were stably dispersed in a distilled water for as long as 24 hours. They were separated from the water by a magnet in two minutes. Since the yield of the flow tubular reactor method is easily multiplied by multiplying the number of the reactors, producing ferrite particles by the flow tubular reactor method will give new impetus to mass production of ferrite particles.