To quantify the emission of airborne actinomycetes from the wastewater treatment plant (WWTP), the concentration and size distribution of airborne culturable actinomycetes were examined with a six-stage cascade impactor in a municipal WWTP with oxidation ditch process in Xi’an, China from June to July 2011. Simultaneously, characteristics of airborne bacteria were also measured to compare with data of actinomycetes. Similar to airborne bacteria, the concentration and size distribution of airborne actinomycetes were found to vary greatly at different phases of the wastewater treatment process. The mean concentration of actinomycetes in the WWTP ranged from 2139±229 at the sludge dewatering house (SDH) to 902±54 CFU·m^-3 at the effluent outlet (EO). The largest emission source of actinomycete aerosols was detected at the SDH, with 21-fold exceeding the permissible standards recommended in the literature. The particle size distributions showed that similar single-peak distribution patterns appeared for both airborne actinomycetes and bacteria. The count median diameter (CMD) of total actinomycetes and bacteria concentration were 2.3 and 3.2 μm in the WWTP, respectively. Although the total concentration of actinomycetes was smaller than that of bacteria in the present WWTP, the respirable fraction (particles smaller than 4.7 μm in aerodynamic diameter) for actinomycetes (81.9%) was higher than that for bacteria (64.6%). The results obtained in the present research suggest that more attention should be paid to the potential health risk related to actinomycetes in studies on wastewater treatments.