A number of laser transitions in the region of 12-13 μm have been obtained by optical pumping of NH3 molecule using a tunable TEA CO2 laser. By studying several different experimental arrangements, it is shown that the optimum pure NH3 gas pressure at room temperature is about 5 mbar. Moreover, some buffer gasses such as N2, H2 and He were employed and it was found that He is the most efficient gas for which the maximum efficiency up to 8% with the mixture of NH3: He =7:16 was obtained. In this condition, the maximum energy per pulse of 125 mJ was achieved. The tunable operation of this laser was established in two distinct wavelengths of 12.812 μm and 12.08 μm.