Lateral torsional buckling is one of the important criterion for determining the flexural strength in steel beam design. probability of it increases when laterally unbraced length is long. On one hand, lateral bracing should have sufficient stiffness to provide lateral support for compression flange of beam. On the other hand, this support must has enough strength. Composite beams consist of reinforced concrete slab and steel beam. To join the reinforced concrete and steel beam, stud has been used to transfer the horizontal force from concrete to steel beams. In fact, it plays role of lateral bracing for top flange.  Bottom flange is also under pressure in the case of double curvature bending and there is probability of lateral torsional buckling. Common methods of beam lateral bracing are connecting two adjacent beams to each other and connectig bottom flange to concrete slab. In this study a new detail has been introduced for lateral bracing of the beam-slab composite system. In this study, bottom flange is connected to concrete slab by a pair of transverse web stiffeners and its rotation is prevented by a pair of transverse studs. Finite element analysis results show that this detail can prevent  bottom flange from lateral torsional buckling.