The effective atomic number, Zeff and electron densities Ne have been obtained for Amino acids containing H, C, N and O in the energy range 5-1500 keV from theoretically evaluated value of total attenuation cross section. The effective atomic cross sections are the total attenuation cross sections divided by the total number of atoms of all types present in a particular sample. Further, the effective atomic weight is defined as the ratio of the molecular weight of a sample to the total number of atoms of all types in it. The variation of the effective atomic number has been systematically studied with respect to the effective atomic weight. The significant variation of Zeff and Ne is due to the variations in the dominance of different interaction processes in this energy region. The variation of the Zeff values with Aeff is studied at all energies of interest. The Zeff values obtained in the energy region of present interest are related to their respective effective atomic weight. It is found that the ratio Zeff / Aeff was 0.53 for amino acids containing H, C, N and O elements in the energy region studied
