Giardia lamblia is a major cause of diarrheal disease all over the world. It is transmitted via the oral route in the cyst stage which is very resistant to disinfectants commonly used in drinking-water treatment. The present work was designed to assess the ability of radioactive Cobalt to inactivate G. lamblia cysts thus they would not be infective to BALB/c mice.One hundred and twenty mice were used and divided into four groups. The first group included 50 mice, infected with a suspension containing human derived non-irradiated G. lamblia cysts and served as control. The second group included 50 mice, which was the experimental group and was infected with a suspension containing human derived G. lamblia cysts that received a dose of 0.25Kilogray of Cobalt-60. The third group (10 mice) received irradiated water with a dose of 0.25 KGy of Cobalt-60 and the fourth group didn’t receive anything. The infectivity was assessed by stool examination and the presence of trophozoites in the small intestine of inoculated mice. Stool examination was done using three different techniques: simple sedimentation, the Mini Parasep SF® and the rapid Copro-Antigen detection kit. Moreover, histopathological examination of the mice’ small intestine revealed presence of trophozoites in group A but not in group B.The cysts that received the radioactive Cobalt were unable to induce infection in the inoculated mice; while the infectivity of the non-irradiated cysts was 95.5%. Among the three methods used in the diagnosis the most sensitive was the copro-antigen with sensitivity of 95.5 % followed by the parasep with sensitivity of 86.6% and lastly the simple sedimentation with sensitivity of 75.5%.The results of the present study indicate that radioactive Cobalt is very effective against G. lamblia cysts and can be used as a control measure to prevent infectivity.