Background: The International Commission on Radiological Protection (ICRP) has attributed the same relative risk for X and gamma radiations of all energies. Several studies have proven that the biological effect of low energy photon is more than that of higher ones. The assessment of risks is important due to the wide use of low energy X-rays for mammography screening and other diagnostic applications.  Materials and Methods: Five X-ray beam qualities characteristics according to ISO-4037 and consultative committee of ionizing radiation (CCRI) were studied in details. In pilot study made to investigate the minimum dose that cause measurable biological effects, one hundred male albino rats were equally divided into five groups namely A, B, C, D and E. Group A was used as control while animals of other groups were whole body exposed to different radiation doses from 137Cs γ-rays. For studying the biological effect energy dependency, eighty male albino rats were equally divided into 8 groups namely F, G, H, I, J, K, L and M.  Group F was used as control and was not exposed to any type of radiation while animals of other groups were whole body exposed to the same dose of radiation but at different dose rates and different energies. Blood samples were collected and serum samples were separated for further biochemical investigations. Biochemical investigations for blood sera included alanine aminotransferase (SGPT), aspartate aminotransferase (SGOT), blood urea nitrogen (BUN), creatinine (CREA), calcium (Ca++) and Creatine phosphokinase (CPK) level.  Results: The results showed remarkable variation of the measured biochemical parameters levels with different photon energies that reflect the energy dependency of the investigated parameters. Conclusion: It’s clear from the data that the same absorbed dose delivered to the exposed animals induces different effect according to the used photon energy. Low energy X-rays were found to be more biologically effective than higher ones.