S.typhimurium is a gram negative bacterium causing serious enteric or extraintestinal infections in human, cattle and birds. These kinds of infections may lead to some permanent or nonpermanent complications like endophthalmitis, Guillain-Barre syndrome and osteomyelitis . Multi-resistant strains of S.typhimurium have been reported from several countries, and these infections are a major public health problem throughout the world. The aim of the present study was to develop a conjugate vaccine for prevention of S.typhimurium infections. For this purpose, polysaccharide side chain was conjugated with tetanus toxoid by carbodiimid mediated amidation method. Immunodiffusion was performed in agarose gel. Four groups of BALB/c mice were selected and immunized with heat killed S.typhimurium, purified polysaccharide, polysaccharide side chain-tetanus toxoid conjugate and sterilized distilled water and the role of each antigen in protection against infection with pathogenic S.typhimurium were investigated. Animals were challenged by intraperitoneal injections. The 50% lethal dose (LD50) was determined on the 21st post challenge day by Reed and Muench method. Immunodiffusion results showed that the conjugate reacted with heat killed whole cell vaccine antiserum and tetanus toxoid antiserum. Three injections of conjugate caused protection against intraperitoneal challenge. In conjugate immunized group LD50 increased 15.4 and 11.11 fold in comparison to control group and polysaccharide side chain immunized group respectively. Results showed that polysaccharide side chain-tetanus toxoid conjugate injected in 3 doses is very protective in mice and can increase the LD50 of immunized animals significantly in comparison to both O antigen vaccinated and control groups.