In this paper, a novel approximation method for conduction loss model of half-bridge LLC resonant converter operating below the resonant frequency of resonant inductor and capacitor. In the conventional conduction loss model, only the fundamental component of switching frequency is considered. However, the disregarded harmonics of switching frequency cause considerable deviation. In the novel method, harmonics of switching frequency are included, and it is more precise than conventional model as switching frequency is far from resonant frequency. This method estimates conduction losses according to actual current waveforms instead of calculating amplitudes of harmonics. Also, switching loss and other loss models are derived and described. Novel loss model and conventional loss model are compared with simulation results.