The behavior of ignition and flame-holding of various plasma torches in supersonic flow were numerically investigated. To understand the role of radicals involved in the plasma jet (PJ), three kinds of PJs (O-2, N-2, and Ar) were simulated. For fuel injection both upstream and downstream of the PJ, the three PJs showed no difference with respect to the location and the behavior of ignition, Ignition occurred at the upper part of the rear jet, where the front jet turned upward because of passing through the interaction shock wave collided with the rear jet first. Differences appeared in the behavior of flame-holding after ignition in the case of upstream fuel injection. The flames ignited by the N-2 PJ and At PJ blew out, but the one ignited by the 0(2) PJ was held by the O-2 PJ itself. This result suggests that the local equivalence ratio is an important factor for flame-holding by the PJ. Moreover, flame propagation in the merged low Mach number region between the PJ and the H-2 jet was observed in cases of the N-2 PJ and the O-2 PJ. On the other hand, there was no significant difference in three PJs in the behavior of the flame in the case of downstream fuel injection. In this case, reactions occurred mainly along the thin mixing layer of the PJ and the H-2 jet, The performance of the At PJ was worse than those of the O-2 PJ and the N-2 PJ for all simulations. Therefore, radicals included in the PJ enhanced the combustion reaction and enlarged the combustion region.