Abstract: It was pointed out that the definition of energy velocity has clear meaning only for a ray of electromagnetic waves with definite frequency and wave vector. Then, some explanation and proof were provided.  By calculating the average energy velocities and the phase velocities of electromagnetic waves in boundless linear media and typical transmission lines, it arrives at the conclusion that：when the electromagnetic wave propagates through linear media by TEM mode, the average energy velocity equals the phase velocity; if electromagnetic waves are transmitted by the TEmn mode or TMmn mode through a transmission line, the average energy velocity is variable on the wave front, and it is always lower than the phase velocity or the speed of light in the vacuum.

Key words: a ray of electromagnetic waves; energy velocity; phase velocity ; transmission lines; transmission mode