A mathematical model is suggested for the approximation of the monoenergetic gamma-ray spectra obtained by a Ge(Li) detector. Some features of the instrumentation monoline spectra in the energy range upto 2 MeV are considered in detail. Computer program “APPROXIMATSIYA” is described. The program allows to determine the parameters of the mathematical model with the purpose to construct a making up standard spectrum library. The results of the monoline spectrum processing are discussed.