Abstract: Optimization of the operating parameters of OPFIRL were studied theoretically. The output power density of the OPFIRL was calculated by solving the density matrix equations of the quantum system without the assumption of Bs(?)0. The theoretical result of the output submillimeter wave power density was in good agreement with the experimental result as reported in the literature. On the basis of these results, the optimum value of pumping power density, length of sample tube and the optimum operating gas pressure of OPFIRL were determined. These optimum operating parameters of the OPFIRL were restricted with each other. By using these results, it is possible to design an OPFIRL operating under the optimum condition by theoretical calculation.