Abstract: A stiffness method for determining the response of shear lag effect on box girder is presented. The differential equations, based on the vriational principles,are derived by assuming a refined longitudial displacement function in equilibrium for the forces in the axial direction. The formulation of the stiffness matrix as well as the equivalent nodal forces can be used to analyze shear lag problem easily Some analytical results obtained by this method are compared with test results and theoretical values obtained by other methods.The major advantage of the present method lies in the great applicability.