This paper presents the experimental results for a streamlined twin box girder of 1545 m span suspension bridge in order to investigate the Reynolds number sensitivity to aerodynamic force and pressure coefficients. High speed wind tunnel testing was carried out on a 1:30 scale sectional model at an aeronautical wind tunnel. The drag and lift coefficients revealed significant decreases at a critical Reynolds number that was in the range of ordinary wind tunnel tests. The safety rail reduced the Reynolds number dependency of aerodynamic force coefficients at negative angles of attack. Similarly, the boundary layer trip strip attached at the bottom surface of the girder reduced the Reynolds number dependency by fixing the flow separation. The pressure coefficients near the gap between the twin boxes changed from negative to positive at the critical Reynolds number, which represented smooth ventilation of wind flow through the gap at the supercritical region. The imbalance of the drag and lift forces acting on each box in the subcritical region gradually resolved and both boxes were almost equally loaded by the forces at the supercritical region. (C) 2014 Elsevier Ltd. All rights reserved.