The disinfection of IP cassettes used in hospitals is important for the prevention of hospital infections. This study intended to examine the safety of a disinfection method using gamma rays, not alcohol that is commonly used in hospitals, in relation to pathogenic microorganisms. Observations using an electron microscope showed vivid cytoplasmic membrane before the irradiation. In terms of a comparison between the test group and control groups, S. hominis showed morphological changes after the gamma irradiation. In other words, the surface of its cell membranes was found to have bursted and been damaged to some extent. However, in the case of gamma irradiation at 5.5 kGy, its cell membranes were observed to have bursted and been entirely destroyed. When each amount of gamma irradiation was fixed at 5.5 kGy through flow cytometry and the bacteria were compared before and after the irradiation, an analysis on the gamma irradiation of B. cereus exhibited that when the number of events was 5,643, viable cells accounted for 66% of its total cells and dead cells accounted for 34%. In addition, B. cereus showed sensitivity with a relatively higher level of permeability under gamma rays compared to S. haemolyticus and S. hominis. The above observations suggest that studies should be continued on the comparison of sensitivity gaps according to the gamma irradiation of each gram-positive bacterium and gram-negative bacterium.