Horizontal curved box bridges are considered important structures due to their high efficiency and economy. It is necessary to know its behavior in general and the effect of changing the box section in terms of shape and dimensions because of the important effect on the behavior and thus the design. The large value of box girders height makes behavior different from the shallow ones because they are subject to the behavior of deep members. A horizontally curved specimen was cast and experimentally tested. It was also numerically modeled using the finite element CSI bridge software, comparing its experimental results with the modeling. The effect of changing the cross-sectional shape, height, and width on the deformation, shear, torsion, and bending moments was numerically studied. The shape of the rectangular section was changed to a clipped, trapezoidal, and circular besides changing the section height and width. From the finite element results, it is clear that the circular shape is the most efficient because it provides an equal distribution of stresses around the center, which makes it more efficient in resisting shear and torsion. Increasing the height by 100% and 200% leads the deflection to decrease by 75% and 86%, respectively. In addition, increasing the width by 15-32%, increases the section's resistance to bending by 15-46%, making the box girder more rigid. Increasing the height led to the direct transfer of stresses from the loading to the supporting points, and thus the deflection and bending moments decreased, accompanied by an increase in shear forces. Increasing the overall width of the girder means there is more zone for shear stresses to distribute.