In shallow water with a thermocline, the characteristics of sound propagation strongly depend on the signal frequency. When only one of the source and the receiver is above the themocline, it is known that the intensity of the received signal changes largely and almost periodically as the signal frequency varies. This is the so-called channel resonance. By using the ray-mode approach, the formula relating the resonance frequency and the sound speed profile is obtained, and the resonance phenomenon is analyzed. Also this analysis is verified by numerical calculation.