Bioaerosols become a more noticed and important problem in indoor air quality (IAQ) control. In this study, we investigated antibacterial effects of silver nano-particles on Escherichia coli, the common Gram negative bacteria and Staphylococcus aureus, the well-mown Gram positive bacteria under aerosol conditions. The bioaerosols containing each bacterial culture were contacted with silver nano-particles sprayed in a closed chamber. Experimental results showed that the silver nano-particles had strong antibacterial activity against E. coli and S. aureus, respectively. As anticipated, high antibacterial activity was found at a high silver concentration and a long the contact time. It was also found that the bactericidal rate decreased with time due to the aggregation of silver nano-particles. Overall, the experimental finding suggested that silver nano-particles could be successfully applied to improve indoor air quality.