A　high-throughput　(105.5 g/h)　passive　four-stage　asymmetric　oscillating　feedback　microreactor　using　chaotic　mixing　mechanism　was　developed　to　prepare　aggregated　Barium　sulfate　(BaSO4)　particles　of　high　primary　nanoparticle　size　uniformity.　Three-dimensional　unsteady　simulations　showed　that　chaotic　mixing　could　be　induced　by　three　unique　secondary　flows　(i.e.,　vortex,　recirculation,　and　oscillation),　and　the　fluid　oscillation　mechanism　was　examined　in　detail.　Simulations　and　Villermaux–Dushman　experiments　indicate　that　almost　complete　mixing　down　to　molecular　level　can　be　achieved　and　the　prepared　BaSO4　nanoparticles　were　with　narrow　primary　particle　size　distribution　(PSD)　having　geometric　standard　deviation,　σg,　less　than　1.43　when　the　total　volumetric　flow　rate　Qtotal　was　larger　than　10 ml/min.　By　selecting　Qtotal　and　reactant　concentrations,　average　primary　particle　size　can　be　controlled　from　23　to　109　nm　as　determined　by　microscopy.　An　average　size　of　26　nm　with　narrow　primary　PSD　(σg = 1.22)　could　be　achieved　at　Qtotal　of　160　ml/min.　©　2022　American　Institute　of　Chemical　Engineers.