This paper presented the results of experimental measurements carried out by E-PERM technique to evaluate Radon concentration in ENRRA research building in Cairo, Egypt to assure that the indoor air quality has no pollutants from Radon gas and in the acceptable international level for work places. . The radon concentrations are measured in 89 locations distributed in the building, and different factors which may influence the Radon concentration are studied; such as human behavior, floor location, orientation, opening size, type of ventilation, function of spaces, and finishing materials. The results has shown that the concentration of Radon gas ranged from less than 0.4 to 37.9 Bqm-3 which is less than the upper reference level by ICRP. Radon concentration is not dependent on the finishing materials and orientation of the spaces but depends on the type of ventilation (naturally or mechanically) and the size of the windows. Finally, the human behavior of the occupants for ventilating the spaces is very significant.