We have demonstrated a scalable approach to synthesize single-walled carbon nanotubes (SWCNTs) with selected diameter distributions by applying a magnetic field perpendicular to the electric field in the arc plasma. It is found that the purity and orientation of SWCNTs can be controlled by the magnetic field. SWCNTs with different diameter distributions can be separated into two different regions by the applied magnetic field, and the diameter-selection efficiency is improved by modifying the direction of the magnetic field. Our findings suggest that the motion of the catalyst particles with different sizes, positive carbon ions and electrons are significantly influenced by Lorentz forces, resulting in the difference in the growth processes of the SWCNTs due to the collective interactions between the arc plasma and the magnetic field.