Two Experiments were carried out in 1999 season. The first was at the experimental farm of Rice Research and Training Center (RRTC),Sakha, whereas the second was at the experimental farm of Gemmiza Research Station. The study aimed to investigate the effect of rice blast disease on yield losses at different growth stages of cvs. Giza 171, Reiho and Giza 176. The cultivars were evaluated in split- plot design as a main plots, wheras the treatments (protection by Beam at the rate of 100g/ fed) at both tillering and heading stages, artificial inoculation with spore suspension of Pyricularia grisea (5×10⁴ spores/m²( at tillering and  or heading stage (milking, soft dough) and natural infection were allocated as Sub-plots. The highest severity and area under disease progress curve (AUDPC) for leaf blast infection were obtained from artificial inoculation at tillering stage. Also, the highest severity and AUDPC of panicle blast infection were recorded from artificial inoculation at milking stage. Giza 171 was the highest susceptible cultivar during vegetative stage, while Giza 176 was the most susceptible cultivar during heading stage. Maximum actual loss due to blast infection in grain yield and 1000- grain weight was obtained from artificial inoculation at milking stage. Panicle blast infection had stronger influence than leaf blast infection. Whereas yield losses due to panicle infection of Giza 171 were two folds compared to those due to leaf infection. These losses were three and four folds in case of Reiho and Giza 176.