Because of increasing incidence of microorganism infections, there are concerns in engineering control of indoor microorganism for reducing airborne infectious diseases. Ultraviolet germicidal irradiation (UVGI) was considered to be promising to inactivate microorganisms. In this investigation, the influences of UV dosage, microorganism species, and relative humidity on UVGI effectiveness were evaluated in a laboratory test chamber. A Collison nebulizer generated aerosols containing Escherichia coli, Bacillus subtilis spores, cells of Candida famata var. flareri, and spores of Penicillium citrinum. The UVGI control effectiveness was determined as the ratio, NS/N0, where NS and N0 were the colony concentrations collected by Andersen one-stage samplers at UVGI dosages of D and zero, respectively. In regard to 99% microorganism inactivation, the UVGI dosage ranges were 1017 to 2356 W s/cm2, 15949 to 19345 W s/cm2, 12917 to 17497 W s/cm2, and 47984 to 89419 W s/cm2 for E. coli, B. subtilis, yeast, and P. citrinum, respectively. Significantly, the microorganism susceptibilities of E. coli were the highest and P. citrinum were the lowest. In regard to relative humidity effects, it was observed that the microorganism susceptibilities at 80% relative humidity were lower than those found at 50% relative humidity for all 4 types of the evaluated microorganisms.