Abstract This study illustrates the mechanism of modal coupling in cantilever plate flutter using the full Theodorsen airfoil theory within the linear framework. An accurate, pseudo-spectral method is employed to calculate the fluid loading and the eigenvalue problem is solved numerically following the Galerkin procedure. For plates with a structure-to-fluid mass ratio around unity, the first two in vacuo modes are dominant and the Kutta condition at the trailing edge plays a central role in the flutter mechanism. The fluid loading induced by the first mode excites significant second and higher order modes. The fluid loading on the second mode is coupled strongly with the structural vibration velocity of the first mode, which is identified as the main mechanism of energy transfer from flow to plate. It is demonstrated that the response of the second mode is suppressed and the plate is stabilized when a concentrated mass is added near the middle of the plate length. Theoretical prediction is supported by experimental data although the latter is affected by many practical factors that are difficult to model precisely.