The current study was designed to examine the antibacterial effect of thyme (thymus vulgaris) on Escherichia coli in diarrheic calves and also the effect of the thyme as immunostimulant. Collection of 100 rectal swabs from 100 diarrheic calves was done for isolation and identification of E.coli and serotyping of the bacterial isolates followed by their antibiotic sensitivity testing and determination of MIC and MBC of thyme oil on them. Supplementation of thyme extract to friesian calves milk was done and examined its effect of on their immunity by antibodies titration of IgM, IgG and IgA in their serum. Results revealed that the incidence of E.coli was 58% and pathogenic E.coli was 46%.The most predominant serotype was O125. The highly resistance of the strains was against oxytetracycline (OT) and ampicillin (AMP). The thyme had antibacterial effect against E.coli strains by MIC ranged from 5 to 320 µg /ml but the MBC was negative for all strains and the thyme extract at 40 mg/kg body weight improved the immunity as it significantly (P≤0.05) increased IgM, IgG and IgA titre in calves serum.