Several authors have mentioned calculations of the effective atomic numbers (Z(eff)) and effective electron density (N-eff) on amino acids, carbohydrates, and nucleotides. Recently, numerous studies performed for determination of the mass attenuation coefficient (mu/rho), Z(eff) and N-eff on some biological compounds. However, theoretical studies for Z(eff), N-eff, and mu/rho for any of chemotrapy drugs is not available in the literature. For 11 different chemoterapy drugs, which are cisplatin, carboplatin, oxaliplatin, ifosfamide, gemcitabine, fluorouracil, pemetrexed, etoposide, vincristine, tamoxifen,and paclitaxel, by applying XCOM program we calculated the mu/rho values, and from these values, the Z(eff) and N-eff values have been acquired.Z(eff)N(eff)Z(eff) The minimum Z(eff) and N-eff values were seen at approximately 0.1 MeV < E < 3 MeV energies where Compton scattering is dominant. When molecules are compared, we can see that Z(eff) reaches the highest values for cisplatin, carboplatin and oxaliplatin that contain Pt element and a minimum Z(eff) value is obtained for Vincristine. Because the N-eff is related to Z(eff), the curves of the N-eff values on graphic exhibit same properties with Z(eff) values. It is thought that Z(eff) and N-eff values presented in this study should be known for use in various medical applications such as radiology, radiation oncology, etc.