The particle image velocimetry (PIV) technique was successfully applied for measuring the normal component velocity of a He II thermal counterflow jet. Neutrally buoyant hydrogen-deuterium solid particles were used as tracer particles for the PIV measurement. The jet velocity profiles and the decay properties were investigated for the cases of small and large normal component velocities, and were compared with those of fully developed turbulent round jets of ordinary viscous fluids. The velocity measured near the jet nozzle exit was compared with the theoretical prediction for the normal component velocity, Un,theo. It is found the ratio of the PIV velocity to Un,theo is smaller than unity at any temperatures irrespectively of Un,theo