The effect of an addition of a small amount of HO2 on ignition enhancement by NOx catalytic reactions was numerically and experimentally investigated. Numerical simulations showed that the addition of a small amount of HO2 to NO or NO2 drastically reduced ignition delay times of H2, CH4, and C2H4 fuels. The existence of the HO2 lowered the threshold pressure at which the NOx catalytic effect significantly appeared. To confirm the effect of the HO2 addition, ignition tests by an O2/N2/H 2 PJ in a supersonic flow (M=1.7) were conducted. However, the effect of the co-existence of HO2 with NOx was not verified in the experiment. The ignitability of an O2/N2/H2 PJ was almost the same as that of the O2/N2 PJ. The reasons for this result were a low static pressure and a low concentration of the HO2 in the PJ plume. Moreover, the effects of radicals and NOx additions to a C3H8 fuel were numerically investigated and ignition tests of C3H8 by the PJ were conducted. The results showed that a supply of radicals and NOx by the PJ was less efficient for C3H8 fuel than for the H2 and small hydrocarbon fuels such as CH4 and C2H4.