Radiative transport of energy plays an important role in many plasma processing devices. Jets of thermal plasma are produced in plasma generators with electric arcs. The experimental observation of radiation transfer is very difficult due to experimental conditions, In such cases, the mathematical modeling is of great importance. Several approximate methods have been described; one of them is the method of partial characteristics. The preliminary task for computation of partial characteristics is the knowledge of absorption coefficients of the plasma. In addition to atomic and atomic ions species, molecules play an important role in re-absorption of radiation in colder arc regions. In the present paper, attention is given to the calculation of photoabsorption cross sections of diatomic molecules. The cross sections of O-2, N-2, H-2, NO, OH, N-2(+), NO+ have been calculated using an approximate method. Results of the calculation are compared with the data available in literature.