In this study, one hundred stool samples were collected from children under age of five years of both sexes suffering from diarrhea infection cases from Al-Kadhimia and Al-Elwia hospitals in Bagdad governorate, and from Ibn-Ghaswan hospital in Basra governorate. Stool samples were cultured on Salmonella-Shigella agar plates, and then 38 bacterial isolates were obtained. All these isolates were subjected to morphological, microscopically examinations and biochemical tests. Results showed that 10 isolates were Salmonella tytphimurium, which further identified using Api-20E and antisera test. Each of these isolates injected intrapretonally in mice to select the most virulent isolate, that capable to kill the animals in the shortest period. Results showed that Salmonella typhimurium SM9 was the most virulent isolates capable to kill the animals within 19 hours after injection in mice. Ability of SM9 resist antibiotics was examined, and it was found that this isolate was able to resist ampicillin, amoxicillin, gentamycin, streptomycin, tetracycline, cephalexin, nalidixic acid, and kanamycin; while it was sensitive to rifampicin, clindamycin, carbencillin, tobramycin, and trimethoprim. Ability of SM9 to produce hemolysin and sidrophore was also examined, and it was found that this isolate was sidrophore producer; while it was unable to produce hemolysin. Plasmid profile of SM9 was studied by extraction the total DNA by alkaline lysis method, then electrophoresis on agarose gel. Results showed that this isolate harboring a large plasmid (mega plasmid) that may be confer to some virulence factors. Curing of plasmid DNA was achieved using ethidium bromide to know the role of this plasmid in the virulence of this isolate, and its ability to antibiotic resistance. It was found that this plasmid was carrying genes conferring resistance to ampicillin, amoxicillin, gentamycin, tetracycline, and kanamycin, in addition to the genes coding for virulence that responsible for the pathogenecity, because of inability of the cured cells to kill mice that injected, rather than its inability to affect the mice liver functions. Liver ability to produce GPT and GOT did not affected blood stream in comparison with control animals. While the wild type (virulent isolate) cause liver dysfunction in lever of mice that led to increase the levels of both enzymes in blood stream of mice, the activity of GPT was increased from 199.3 U/L (in control animals) to 375.1 U/L in injected mice with wild type cells SM9, also the activity of GOT was increased from 68.0 U/L (in control animals) to 90.8 U/L in mice injected with the wild type cells of SM9. In order to know the ability of conjugative plasmid of SM9 to transfer to member of other Enterobactereaceae, conjugation on solid medium by modified filter mating procedure was achieved between SM9 (donor) and the standard strain E. coli MM294 (recipient), and after culturing on selective medium, number of transforming cells were received the antibiotic resistant traits to ampicillin, amoxicillin, gentamycin, tetracycline, and kanamycin. Plasmid profile of transconjugants was examined, and it was found that there was a plasmid band after electrophoresis on agarose gel, in addition to chromosomal band in comparison with the wild type of E. coli MM 294, that refer to virulence plasmid of SM9 isolate was a conjugative plasmid.