The aim of this study was to evaluate antibacterial activity of aqueous extract of castor seeds (two varieties, Mashhad and Isfahan) on Staphylococcus aureus ATCC25923, Escherichia coli ATCC25922 and Listeria innocua ATCC33090 as food borne pathogens. The sensitivity of the microorganisms was evaluated using disc diffusion, minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC). According to disc diffusion method the most resistance was observed by gram positive bacteria (Listeria innocua, Staphylococcus aureus) where as, the most sensitivity was observed by Escherichia coli.In disc diffusion method as a positive control Erythromycin, Gentamicin and chloramphenicol were used on Listeria innocua, Staphylococcus aureus and Escherichia coli in which deterrence diameters were 13mm, 22mm, 30mm, respectively. The experiments of MIC, MBC were performed in triplicate. According dilutions which were prepared in MIC experiment the ranges of aqueous extract of castor from Mashhad on Listeria innocua and Staphylococcus aureus was between 40 to 160 mg/ml. This range for Escherichia coli was less than 40 mg/ml. But for aqueous extract in castor seeds from Isfahan was between 20 to 40 mg/ml on Listeria innocua, between 40 to 80 on Staphylococcus aureus and between 20 to 80 mg/ml on Escherichia coli. Also MBC test were measured around 160 mg/ml in castor seeds of Mashhad and Isfahan. The both of aqueous extract of castor seeds from Mashhad and Isfahan had strong antibacterial activity and aqueous extract from Isfahan had more inhibitory effect than the aqueous extract of Mashhad.