Thermal plasmas are a promising tool for high-throughput production of nanopowder because thermal plasmas offer a very-high-temperature field with steep gradients at their fringes where many small nanoparticles are generated rapidly from the material vapor. The fringe of a thermal plasma flow induces vorticities by fluid-dynamic instability and the vortices transport the growing nanopowder cloud by turbulent convection. Meanwhile, a magnetic field whose direction is the same as the plasma’s main flow significantly constricts the plasma to the central axis. This study investigated the controllability of an axial magnetic field on nanopowder cloud growing and transported in a turbulent field induced by a thermal plasma jet using time-dependent three-dimensional simulation with the numerical method suitable for thermal plasma flow systems. An argon plasma jet was ejected with the maximum temperature of 12,000 K and the maximum velocity of 400 m/s. Silicon vapor is conveyed at 0.1 g/min with the plasma jet. As numerical results, Fig. 1 shows that the plasma jet with a magnetic field was longer and less fluctuated than that without a magnetic field. Figure 2 shows that the nanopowder cloud formed a thinner shape with larger particle sizes when the magnetic field was applied because of the suppression effect on turbulent vortices.