Both modal expansion and travelling wave methods are commonly used for predicting the response and vibrational energy flow in structures. They describe the same structural wave motion problem from different viewpoints. In this paper, energy flows carried by the torsional and flexural waves in beam structures are predicted by these two methods and the results are compared. It is shown that the convergence property of the energy flow predicted by the modal expansion method may be poor in the vicinity of the forcing location. The limitation of using a finite number of continuous mode shape functions to represent the discontinuity at the forcing location is illustrated. It is observed that the energy flow predicted by the modal expansion method oscillates about the exact energy flow obtained by the travelling wave method. The sources of the fluctuation are investigated. The study also suggests that spatial averaging of the oscillated energy flow may give a better approximation of the exact energy flow without using an extremely large number of modes.