Giardia lamblia is the main cause of the water borne diarrhea (Giardiasis) in cattle and dogs as well especially the young ages. We developed an experimental mice model for G. lamblia infection. The infection was performed by oral administration of either G. lamblia cyst or trophozoites (5 x106/mouse). The fecal cysts and the body weight of the mice were monitored for 15 days. The results showed that the trophozoites-infected group showed higher fecal cysts count and body weight loss. These results indicated that infection with trophozoites showed higher pathogenic effect in mice. We developed anti-G. lamblia IgY (anti-GL IgY) in chicken egg yolk. The in-vitro and in-vivo efficacy of anti-GL IgY were investigated in both intestinal cells and the mice model. The in-vitro experiment showed that the number of the attached trophozoites to the intestinal cells was reduced significantly in the anti-GL-treated cells in comparison with the control IgY-treated cells. This result indicated that the anti-GL IgY reduced the attachment ability of the trophozoites to the intestinal cells. The mice were received anti-GL IgY in the food (5%) from the day 0 of the experiment. The experimental infection was done by administration of 106 trophozoite/mouse. The body weight and the fecal cyst were monitored for 40 days. The results showed that there were significant differences in the body weight loss and the fecal cyst count between the control IgY-treated group and the ant-GL-IgY treated group. These results indicated that the anti-GL IgY reduced the pathogenicity of Giardia lamblia in the mice. Therefore, anti-GL IgY can be considered as a prophylactic immunotherapy or possibly an adjunct to antiparasitic therapy against giardiasis.