Multidrug-resistant bacterial strains are becoming a serious problem, therefore, application of newer antimicrobial agents, as the plant extracts as a natural antimicrobial source combined with antibiotics as tried to overcome this problem. In-vitro methods as the Decimal Assay for Additivity (DAA) is being applied to define the end point for additivity, as the interactions are known on a mechanistic basis to be either additive or synergistic or antagonistic. The present study was carried out to evaluate the antimicrobial activity of five plant extracts prepared by the ultrasonic-assisted methanol extract (UAE) namely Rosmarinus officinalis L. (Rosemary), Origanum majorana L. (Marjoram), Mentha spicata L.(Mint), Anethum graveolens L.(Dill) and Azardirachta indica L.(Neem);combined with antibiotics like amoxicillin, doxycycline, gentamycin and difloxacin against 120 isolates from poultry, among them 79 isolates were belonging to E.coli by using agar diffusion method. The mean zones of inhibition (mm) of methanol plant extracts and antibiotics were determined at different concentrations; also the minimum inhibitory concentration (MIC) of the plant extracts or the antibiotics or combination between them was determined by the twofold dilution method; while the antimicrobial activities were assessed by using disc diffusion method.Total phenolic content (TPC) of plant extracts was determined by the Folin–Ciocalteu method.Also the antioxidant activity of the extracts was determined by the 1,1-diphenyl-2-picryl-hydrazyl (DPPH) assay. Results revealed synergistic effects appear in rosemary with amoxicillin by ratio (7:3),(6:4) and gentamicin by ratio (7:3),(6:4) and difloxacin with ratio (7:3),(6:4),(5:5) while Dill and amoxicillin by ratio (7:3) and gentamicin by ratio (7:3),(6:4),(5:5) and Peppermint with doxycycline only with (7:3),(6:4),(5:5).