Flutter running waves in turbine blade cascade were often observed at experimental investigation of blades vibrations, but theirs detailed analysis has not yet been sufficiently realized. This contribution tries to explain origin and behavior of running flutter waves in the rotating blade cascade excited by steam flow from the stationary bladed disk. One of possible reasons for existence of running waves are the running periodic forces from steam wakes due to different numbers of blades of stationary and rotating wheels. Properties - velocities, directions and modes - of these forced running waves are shown using computational model of turbine disk with 10 blades. Interaction of this kind of forced excitation with aero-elastic self-excitation - flutter - causes origin of the flutter running waves. These flutter waves, modelled by the Van der Pol formula, are analysed in the paper for this simplified form of aero-elastic self-excitations.