Based on the density matrix theory of quantum system, the spectra of optically pumped superradiant and cavity NH 3 FIR lasers and their optimum operating gas pressures are calculated by means of iteration method. The calculation shows that, compared with the optically pumped superradiant laser, the cavity laser behaves a wider band spectrum of FIR emission, a lower optimum operating gas pressure and a higher total output power under certain condition. Moreover, the cavity laser has characteristics of longitudinal tuning. The FIR laser emissions of the superradiant and cavity NH 3 lasers pumped by TEA CO 2 10R(8) and 9R(16) are measured experimentally, the results are in good agreement with the calculation.