Abstract: In order to study the effect of heat stress on the antibacterial resistance and plasmid profile in Escherichia coli, thirty E. coli were isolated from sheep liver. Antibiotic susceptibility test were done by antibiotic disc diffusion method using filter paper disc on two 24 h cultures of each isolate which grown at 37 and 43°C simultaneously in BHI Broth (Merck VM460193 531). The isolates which grown at 43°C were under heat stress during their growth. Ten commonly used antibiotics, viz., ampicillin, erythromycin, neomycin, trimethoprim-sulfamethoxazol, lincospectine, tetracycline, gentamycin, flumequine, vancomycin and Tiamulin (Padtan Teb). The resistance level of all E. coli isolates against 10 antibacterial drugs compared statistically in 37 and 43°C using MINITAB Version 14 program. Plasmid DNAs were extracted from each of the E. coli isolates which were grown at 37 and 43°C overnight using alkali lysis method. In this study ·λDNA (EcoR1+Hind III digested) was used as marker DNA. According to the results of this study, the resistance rate of E. coli isolates have decreased against trimethoprim-sulfamethazol, lincospectine, tiamaulin, tetracyclin and gentamycin at 43°C but only the difference between the resistance rate against gentamycin in 37°C (83.3%) and 43°C (60%) was significant Characterization of Plasmid DNAs by agarose gel electrophoresis showed that each of the thirty drug resistant E. coli harbored a single plasmid. There was no difference among the plasmid profiles of the thirty isolates in 37 and 43°C. As the plasmid profile did not change in 43°C (heat stress) so the resistance differences against antibacterial drugs were not significant except for gentamycine that its resistance may is chromosomal. According to the results of this study, In conclusion it can be said that heat stress could not be effective on antibacterial resistance and plasmid profile if the duration of the stress is short. The long duration of the heat stress plus other stress factors such as starvation will effect the plasmid replication and finally plasmid copy number of bacteria. Mechanism of this phenomenon remains unknown, though one might speculate that some bacterial addiction modules that are activated upon amino acid starvation, like mazEF could be involved.