Background & Objectives: In the nuclear industry, aerosols play an important role on emission and dispersion of pollutants. Nuclear reactors are the principal sources for pollution in fuel cycle and emit different radioisotopes into the environment. The radioactivity and size distribution of aerosols were measured using Anderson cascade Impactor (ACI) at three workstations in Tehran Research Reactor safety enclosure. Methods: Mass concentration and distribution of airborne particles in the chamber environment were determined using ACI. In order to identify radionuclides present in airborne particles of chamber fiber filters were used. Element identification and mass concentrations were performed using ICP-MS. To identify radionuclide type and determine their radioactivity in suspended particles, γ-ray spectrometry and liquid Scintillation spectrometer were used, respectively. Results: Pb212، Pb214، Be212، Be214، Tl208، Ac228 and k40 radionuclides were determined by gamma spectrometry. Maximum mass was related to fine particle of 0 to 0.4 µm. The greatest portion of alpha and beta particle were adsorbed on aerosols with the aerodynamic diameter of less than 0.4µm. Conclusion: Majority of radioactivity was related to the particles in storage and nucleation areas (fine particles). The origin of these particles may be coagulation of mode particles "nucleation" and concentration growth in the particle size of studied.