We study the effect of quantum reflection over the barrier (ROB) in a thermionic cooling device. We find that the performance of refrigerators can be enhanced by the ROB effect if the bias voltage and the lattice thermal resistance of the semiconductor in the barrier region are both sufficiently high. Furthermore, the figure of merit Z T can be higher due to the ROB effect if the workfunction of the cathode is low and the lattice thermal resistance is high. The overall optimum Z T calculated with and without the ROB effect are 6.5 and 7.1, respectively. The origin of the ROB correction to Z T is that the quantum reflection becomes asymmetric for the carrier transport in opposite directions.