Lateral-torsional buckling occurs in steel beams where the section is not adequately supported against lateral movement and twisting . The lateral support can be provided by rigid floors or transverse beams. In this research the effect of transverse stiffeners on the lateral-torsional buckling strength of laterally unsupported steel beam was investigated. Simply supported I steel beam built up section of a length 2.5 m was studied. The length of the beam was chosen between Lp and Lr of the section so that inelastic lateral-torsional buckling can occur. Finite element analysis was performed on the beam without stiffeners and with stiffeners at various locations along the beam's span. The analytical results indicated that the magnitude of the load causing lateral-torsional buckling increases by (3%) as the number of transverse stiffeners increase. The analytical results were validated experimentally by testing some of the beams with different locations of the stiffeners along the beam's span.