This study describes a rational design principle of aerosol reactors for synthesis of high yield monosize ultrafine particles formed through a nucleation-driven particle formation process using the relationships among four characteristic times. A systematic approach to the scale-up of aerosol reactors is then discussed based on turbulent theory, with a simple scaling law being subsequently derived, named the ’cube-root law’. Based on these relationships, a procedure is presented for the design of an industrial scale jet mixing type aerosol reactor.