Measurement of beta radioactive contaminant in the air of medium activity laboratory. Radiometallurgy Installation is used for post Irradition examination (PIE) of nuclear fuel. The examination of spent fuel radio - chemically is done at medium activity laboratory.In this examination, the radioactive contaminant may be distributed in the air of working area. To measure the radioactivity contaminant in the air caused by PIE activity. air sampling time must be determined and sample counting must be decayed in order to eliminate the natural radiactivity. The measurement result shows that the optimum air sampling time is 20-30 minutes and no decay time of air sample (directly)