Abstract: It is commonly accepted that end plates can reduce the influence of flow on both ends to ensure nominally two-dimensional flow in the section-rigid model tests in wind tunnels. One of the governing parameters is the size of the end plate. In order to reveal the effects of end plates on the aerodynamic force distribution on the rectangular prisms which corresponds to the bridge cross-section with three side ratios of 1, 5 and 10, the variations of aerodynamic force, wind pressure coefficients and St with the size of end plates are discussed based on the data from rigid model pressure tests. The results indicate that the free ends of a cylinder have obvious impact on the wind pressure distribution not only near the ends but also in the middle. The influenced region of the cylinder becomes smaller with increasing side ratio. End plates can strengthen the suction on the leeward side. As for the larger angle of attack, the larger end plate is needed to keep the nominally two-dimensional flow. The size of the end plate also has important effect on the Strouhal number of all three models.