Using long Josephson tunnel junctions, dynamic vortex motion has been observed as the flux‐flow voltage across the junction. The flux‐flow voltage increases with increasing external magnetic field. The velocities of the flux flow in the junctions can be obtained from the junction‐voltage–vs–applied magnetic field characteristics. It is shown that the velocities of the flux flow in the long junctions have the maximum value equal to the phase velocity of the electromagnetic field.