Cryptosporidium parvum is a well-known waterborne and opportunistic intracellular protozoan parasite that causes diarrheal illness. In this study, we quantitatively investigated reduction of the infectivity of C. parvum after gamma irradiation and repair of the infectivity during incubation time after irradiation. C. parvum oocysts were subjected to gamma irradiation at various doses (1, 5, 10, and 25 kGy), and the in vitro infectivity was measured by real-time PCR every day up to 7 days after irradiation. The in vitro infectivity of C. parvum on human ileocecal adenocarcinoma cells (HCT-8) was effectively reduced (> $2;{log}_{10}$) by irradiation at 10kGy or more. However, in the experiment to find out repair of the infectivity, recovery was not noted until day 7 post-incubation.