Our study was conducted to describe the pathogenicity of avian pathogenic E. coli (APEC) O78 in infectious bronchitis (IB) virus (IBV) vaccinated broiler chickens. Broiler chickens vaccinated at 1 day old and 14 days old with H120 and IB var2 vaccines respectively were challenged with APEC O78 strain 4 days later. This group was compared with vaccinated/unchallenged, unvaccinated/challenged, and unvaccinated/unchallenged broiler chicken groups. Clinical signs, postmortem findings, body weight, body-weight uniformity, pathological findings, and bacterial count from different tissues which measured 3 and 7 days post challenge (DPC) were used as an evaluation parameters. In our study, IBV live attenuated vaccines (IBV H120 and IB var2) followed by APEC O78 challenge improved colibacillosis clinical signs, pathological findings, and bacterial count in chicken tissues 3 DPC than 7 DPC comparing to unvaccinated/challenged group. However, body-weight uniformity was negatively affected 10 DPC in vaccinated/challenged group comparing to other chicken groups. From an applied perspective, prevention of APEC infection is required to avoid the adverse effect on body-weight uniformity after vaccination with IBV live attenuated vaccines.