Antimicrobial resistance (AMR) has become a serious problem in poultry farms that can threaten both poultry and human health. The objective of this study was to evaluate effects of encapsulated EO as antimicrobial alternatives including cinnamaldehyde (CIN) and citral (CIT) alone or in combination (CIN+CIT) on growth performance, meat quality, gut health, AMR of broiler chicken fecal E. coli, and zoonosis of poultry-source AMR extraintestinal pathogenic E. coli (ExPEC). Chapter one gave a general introduction of the study background. Chapter two provided the literature review with detailed information about poultry farms and AMR. Chapter three described the hypothesis and objectives of the study. Chapter four showed effects of bacitracin (BAC), CIN, CIT or CIN+CIT on growth performance, gut lesions, and cecal microbiota of broiler chickens receiving coccidiosis vaccines or not. The feed conversion ratio , mortality (%), and gut lesion scores were all reduced and cecal microbiota was modulated in birds fed BAC, CIN, CIT, and CIN+CIT compared to birds fed basal diets. Chapter five investigated effects of CIN, CIT, CIN+CIT on AMR phenotypes and genotypes of E. coli in broiler chicken feces. The AMR levels (%) of chicken fecal E. coli to most tested antimicrobials were lower in birds fed CIN or CIN+CIT which also showed reduced prevalence (%) of some antimicrobial resistance genes (ARGs) and plasmids. Chapter six determined effects of CIN on breast meat quality and gut health parameters of broiler chickens. The CIN improved intestinal nutrient digestibility, morphology, gene expressions for nutrient transporters, and changed cecal and ileal microbiota. In Chapter seven, zoonosis of poultry-source AMR ExPEC was evaluated by measuring the survival (%) of Caenorhabditis elegans when exposed to different ExPEC. ExPEC from poultry meat and feces had significant effects on reducing survival (%) of C. elegans but relationships between antimicrobial susceptibility or number of virulence genes with pathogenicity of E. coli were not conclusive. In conclusion, encapsulated CIN has the potential to improve growth performance, gut health, meat quality, and reduce AMR in chicken fecal E. coli. Additionally, ExPEC isolated from poultry meat or feces may possess zoonotic potential to cause human infections.