Avian colibacillosis is an infectious disease of domestic poultry with economic importance.Among the tools available for the control avian pathogenic E. coli (APEC), vaccines have received the most attention. In this research, comparative bacteriological examination between candidates of live aroA mutant E. coli vaccine and two virulent APEC strains (O78 and O125) was carried. Innate immunity (leukogram & Phagocytosis) as well specific cellular (CD4 & CD8) and humoral (agglutinins) immune response of vaccinated chickens were evaluated. Also the mortality rate, colibacillosis lesions and bacterial re-isolation were detected after challenge to stand upon their minimizing in vaccinated chickens. The comparative study demonstrated that the vaccinal and virulent strains were similar in respect of colony morphology, biochemical reactions, Congo red binding and motility but were differ in antimicrobial sensitivity pattern as well inability of vaccinal strain to grow on minimal agar medium. The immune correlate of aroA mutant E. coli vaccine protection is linked to cell mediated immunity not to circulating antibodies. Finally, it could concluded that spraying of aroA mutant vaccine provide virtually protective efficacy in broiler chickens against homologous APEC O78 challenge and had not a  significant values against heterologous strain, although it is safe as alternative to chemotherapeutics.