Background: Bio aerosols include airborne micro-organisms such as bacteria, fungi, viruses, etc and their products. Exposure to a bio aerosol is linked with a broad spectrum of health problems, including infectious diseases, acute toxic effects, allergies, and cancer. The aim of this study was to evaluate the quality and quantity of bio aerosols found in the air of different wards in Valiasr Hospital, Zanjan, in summer and fall 2012.Methods: Air samples were collected from six wards including: operating room, infectious, ear, nose, and throat (ENT), surgery, adult intensive care unit (ICU), oncology and administrative with a single-step Anderson sampler. The type and number of colonies were determined in the laboratory, and then the bio aerosol density were calculated in terms of cfu/m3 and compared with the recommended limits.Results: The most common genera of isolated bacteria and fungi were Staphylococcus and Penicillium, respectively. In the infectious ward bacterial density was higher than the recommended limit of WHO (100cfu/m3) in the visiting times (afternoon). The fungal density in the meeting time (afternoon) in the ICU, ENT, and general surgery, infectious and administrative wards, and in non-visiting times (morning) in the infectious ward was higher than the recommended limit of WHO (50 cfu/m3).Conclusion: From the findings of this study it can be concluded that the density of fungi and bacteria in the hospital air in some times of working period are higher than recommended levels and therefore, the condition of existing air filtration and ventilation systems should be appropriated according to the international standards of hospitals buildings.