The total attenuation coefficients of 11 biological targets against photon energies were plotted within the energy range 1-10 MeV. From these plots a possible equation describing the relationship between photon energy and the total attenuation coefficient was derived. The suggested equation is itilp = 0.0696/root E. To test the validity of the equation and to compare the results of the total attenuation coefficients of 3 tissue-equivalent materials (bolus, rice grains, and boiled rice) that could not be determined before, they were determined experimentally by using X-rays produced at 6 MV and 18 MV. The results of the experiment show a relatively good match with the computed values using the suggested equation. It is possible to conclude that 1) the suggested equation is a practical way to define the total attenuation coefficients of biological tissue or tissue -equivalent materials and 2) the attenuation coefficient is one of the most important to determine the absorbed dose by patients and so they should be worked out experimentally before treatment.