This study investigated viability variation of airborne bacteria in indoor environments. The survival in air as a temporal function of bioaerosol viability was reported for Escherichia coli (KCCM 12119, ATCC 11775). Bacteria suspended in distilled water were aerosolized and entered the vertical duct oriented downward. After measurement of number concentration and colony forming unit (CFU) of the bacteria at different locations of the duct, the viability function was calculated. It was found that the bacteria viability(%) decreased with time after aerosolization, 28.454e-0.132x (x:time, min). This study demonstrated the potential application of viability function of airborne bacteria to studies of exposure assessment and infection risk analysis. Keywords : viability function, E.coli, airborne infection, infection probability