The Maanshan Bridge over the Yangtze River in China is a new long-span suspension bridge with double main spans of 2x1,080m and a closed streamlined cross section of single box deck. The flutter performance of the bridge was investigated via a wind tunnel test of a full bridge aeroelastic model at a geometric scale of 1:211. The test was conducted in both smooth and simulated boundary layer wind fields with various combinations of wind yaw and inclination angles. A unique and interesting flutter-mode transition behavior was observed in smooth flow. As wind speed approaches the flutter threshold, the model oscillation with the first antisymmetric torsional mode (A-T-1) transfers to alternate torsional oscillation between double spans, then transfers to oscillation with the first symmetric torsional mode (S-T-1), and then violent oscillation occurs. The mechanism of the phenomenon is unknown, and there may be no numerical method to simulate it to date. One possible reason for the flutter-mode transition is that a nonlinear aerodynamic self-excited force acting on the middle pylon dragged the full aeroelastic model into soft-type flutter. The other inference is that internal resonance occurred, in which energy transforms between two spans and between modes A-T-1 and S-T-1 at the same time. (C) 2014 American Society of Civil Engineers.