Second-harmonic auroral kilometric radiation (AKR) is discussed within the context of the cyclotron maser theory by Wu and Lee. The results are in good agreement with the ISIS 1 observation reported by Benson. The major conclusions are as follws: (1) when &ohgr;e/&OHgr;e0.3, the fundamental AKR may become weaker than the second-harmonic AKR; (3) when &ohgr;e/&OHgr;e>0.3, the fundamental AKR may diminish; and (4) the second harmonic AKR with frequencies below 2&OHgr;, can propagate downward.