Abstract: The study was conducted to determine growth promoter effects of Effective Microorganisms (EM) on Horro and Fayoumi chickens. This study was conducted at Agricultural Research Center and National Veterinary Institute (Debre Zeit). A total of 450 chickens (225 from each breed) were used in this study. Birds were grouped according to treatment groups: EM-treated (with feed, with water, with feed and water) and non-treated (only SRBC treated and Non-EM, Non-SRBC) controls. EM was given daily from the 3rd week of age for 5 weeks. Daily feed intake and weekly body weight measurements were made. The finding shows that: (1) EM supplementation had no observable effect on mortality, weight gain and FCR in both breeds but Fayoumi had inherently higher body weight than Horro. This study did not consider the cellular arm of the immune response and EM response to infections with specific pathogens was not investigated, collectively demanding further research in these and other issues if EM has to be used as good feed additive.