Radon is the most harmful natural contaminant in the indoor atmosphere of the buildings. The noble gas, after cigarette smoke, is the biggest cause of lung cancer, and today the study of its diffusion, distribution, and concentration around the world has attracted many researchers in the field of radiation protection and environmental health. Typically, output data obtained from traditional methods of measuring radon concentration in indoor buildings is limited to information on the average radon concentration. Although these data are highly valuable in identifying buildings with a high risk of radon, it can be misleading to identify the real danger for residents of these buildings. In this research, by selecting a sample building and using numerical simulation based on the powerful method of computational fluid dynamics, the distribution of radon concentration in several important respiratory levels was investigated. The results indicate a non-uniform distribution of radon concentration in this building and they show that radon concentration in some of the major building sites can be up to 90% higher than the average radon concentration of the whole building. Therefore, in order to calculate the annual absorption dose in many buildings, it is necessary to study the distribution of radon.