The behavior of the optically pumped laser in formie acid vapor at 393, 418, 432 and 513 μm has been studied and the wavelengths were measured by means of Si scanning Fabry-Perot interferometer. The precision was up to 0.4%. The behavior of multi-wavelengths laser of the formic acid-methanol vapor at 118.8, 418, 432 and 513μm has been studied. No significant effect of esterificated aciton of formic acid-methanol on output of the laser has been found under the condition of the experiment.