Upper-room ultraviolet germicidal irradiation (UR-UVGI) is recommended for the defense against airborne pathogens in poorly ventilated rooms in public buildings or high risk environments, such as hospitals. Currently, there are few studies on the performance of UR-UVGI with the effective bacteria susceptibility constant (eZ-value) obtained from UR-UVGI experiment. In this study, the Eulerian model for the inactivation of UVGI was improved to consider the difference between exposure time and computational time, and was validated by our previous experiments. The method was applied to study numerically the performance of the UR-UVGI for 3, 6, and 10 air changes per hour (ACH) with the application of the eZ-value of airborne bacteria. Moreover, the eZ-values of commonly found bacteria, including Serratia marcescens, Staphylococcus epidermidis, Pseudomonas alcaligenes, and Micrococcus luteus, were obtained from a series of experiments in a full-scale environmental chamber equipped with a UR-UVGI fixture. The eZ-values were 0.0983, 0.0586, 0.0476, and 0.0115 m²/J, respectively. Compared with the data in the literature, these eZ-values of the tested bacteria are different from the Z-values obtained from single-pass UVGI. The simulation results show that UR-UVGI exhibited the highest inactivation efficiency on S. marcescens among the tested bacteria in this study. The percentage of bacteria inactivated by UR-UVGI decreased as the ventilation rate increased. The bacteria concentration in the breathing zone under low ventilation rate could be decreased using UR-UVGI. The findings demonstrate that high indoor air quality can be achieved with the application of UR-UVGI without the need to maintain high ventilation rate. Copyright © 2017 American Association for Aerosol Research