Escherichia coli resistance to third generation cephalosporins due to extended-spectrum-beta-lactamase (ESBL) production is a major drug-resistance issue. The present work was undertaken to investigate the phytochemical composition and antibacterial effects of cinnamon bark essential oil (CBO) and its major components individually and combined with antibiotics against ESBL-producing E. coli. CBO was obtained from pharmaceutical source and analyzed by Gas Chromatography/Mass Spectrometry (GC/MS). The antibacterial activities of CBO and its major components against ESBL-producing E. coli were assessed. The effect of combination of either CBO or its most active component with some antibiotics such as co-amoxiclav, ceftazidime, gentamicin, and ciprofloxacin against ESBL-producing E coli were evaluated. GC/MS analysis showed that the major components identified in the CBO were cinnamaldehyde (63.69%), cinnamyl acetate (9.93%), and 1,8-cineole (8.75%). The obtained results indicated that the CBO have highly significant antibacterial activity against all the tested isolates. Cinnamaldehyde proved to be the most active component of CBO, also; combination of this component or CBO with gentamicin resulted in the highest antibacterial activity, although lower dose of gentamicin was used in these combinations relative to that applied when used individually. These findings highlighted the potential role of CBO or cinnamaldehyde as an antibiotic resistance modifying agents. To our knowledge, this is the first report concerning the synergistic effect of CBO and cinnamaldehyde in combination with antibiotics against ESBL-producing E. coli isolated from clinical sources.