The international organizations, including IAEA, have established a diagnostic reference levels (DRLs) for diagnostic xray examinations in order to control doses. The DRLs are mostly expressed as an entrance surface dose (ESD) to quantify dose on the patients. However, there are significant uncertainties associated with the measurements of ESD, and most of them are results from back scatter factor (BSF). The accurate BSF could be determined only by Monte Carlo simulations. In addition, as the human body is much more complicate compared with a commonly used ANSI phantom, for realistic simulation the computational anthropomorphic extended cardiac-torso (XCAT) phantom has to be used in order to obtain accurate BSF. However, to our knowledge, a few BSF values have previously been reported in the literature in x-ray examinations considering patients’ age with XCAT phantom with Monte Carlo methods. The aim of this study was to quantitatively analyze the BSFs using XCAT phantom with Monte Carlo simulation in various x-ray examinations considering the patient’s age. For chest examinations, the BSFs were 1.21, 1.29, and 1.35 for one-year old, seven-year old, and adult patient’s protocol, respectively. For adult patient, the BSFs were 1.35, 1.26, and 1.10 for chest, abdomen, and extremity examinations, respectively. In addition, the ESDs in chest examinations were 36.30 μGy, 52.69 μGy, and 111.80 μGy for one-, seven-year old, and adult patient’s protocol, respectively. These results demonstrated that accurate BSF should be chosen considering the diagnostic examination in order to estimate the ESD properly.