Microbial fauna of the outdoor ambient air of Istanbul has been determined in European side of Istanbul province at four differentdistricts (Bakirkoy, Fatih, Basaksehir and Esenler) by active and passive sedimentation sampling methods. In addition tometeorological factors, the effect of urbanization and traffic density for the air microbial load is also determined. Outdoor airsampling was performed between April 2014 and November 2015. The sedimentation method was done by standart plate count agar(PCA) opening the cover of the petri dishes containing a prepared medium. Standart plate count agar (PCA) medium was used for thedetermination of total bacteria while malt extract agar (MEA) was used for fungi. The colonies formed at the end of this period werecounted and total number of bacteria and fungi per unit volume and area were determined. Air temperature, relative humidity, windspeed and direction were also recorded. As a result of the active sampling method, the most counted units of bacteria was determinedat 35${}^oC$ in Basaksehir station as 222 CFU(Colony forming unit)/$m^3$, maximum fungal was determined as 286 CFU/$m^3$at 20${}^oC$incubation temperature. The maximum amount of bacteria in the passive sampling method results at 35${}^oC$ was in Bakirkoy station as9250 CFU/$m^3$.