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Fifty E. coli strains isolated from chicken flocks were analyzed to determine their resistance to antimicrobial agents used in Tehran poultry industry. By using Mast Diagnostic kit only O6 serotype was identified. Multiple resistance to antibiotics was observed in all isolates. The highest rate of resistance was against Tetracycline (94%), followed by Rifampicin (90%), and Oxytetracycline (80%). Fifty E. coli isolates showed 10 different patterns of resistance to the antimicrobial agents used in this study. The most common antimicrobial resistance pattern of these isolates was Enrofloxacin / Trimethoprim-Sulphamethoxazole / Tiamulin / Flumequine / Sulphadiazine / Oxytetracycline / Rifampicin. The present study confirms significant increase in the incidence of resistance in E. coli isolated from poultry which is probably due to increased use of antibiotics as feed additives for growth promotion and prevention of disease, resistance transfer among different bacteria and possible cross resistance between antibiotics used in domestic animals and those used in human medicine.