Evaluation of the optimal application fields of gamma-rays sources based on radio nuclides 192Ir and 75Se for NDT by means of mathematical modeling is discussed. The first results of using the mathematical modeling for the analysis of the radiographic image quality of penetrameter are given. The optimal application fields of gamma-rays sources for NDT based on this analysis is determined. A discrete model of penetrameter is developed and series of calculations according to these penetrameters is performed. The application field of 192Ir and 75Se isotopes is estimated by using results of these calculations. Numerical investigation of influence of real-time gamma ray spectrum of these radio nuclides is carried out. The influence of scattered radiation on the quality of radiographic images is analyzed.