The shear lag effect on box girders has been studied by quite a few researchers for many years. However, most of studies are related to shear lag formulas and analytical methods and very few have investigated the effect of stress concentration and deflection. The present study investigates the stress concentration and deflection under shear lag effect in a continuous box girder and a simply supported beam by using the three-dimensional finite element method. The whole girder is modeled by shell elements, and an extensive parametric study with respect to the geometry of a box girder is carried out. The influence of finite element mesh on the shear lag is carefully treated by the multi-mesh extrapolation method.