Compare to the straight girders, horizontally girders show complex behavior because torsional moments are always acting on the structure. Because of the torsional moments, there can be negative reaction forces at some supports, and that can cause the overturning of super-structure. Sometimes, this stability problem is more important in the construction stage rather than after construction. However, it is rarely considered in design except special case. In this paper, numerical analysis is performed to investigate the reaction forces characteristic of horizontally curved steel box girder.