The present invention relates to a Switching Mode Power Supply (SMPS) and a driving method thereof. To this end, the present invention provides a SMPS, which is driven in either a Continuous Conduction Mode (CCM) or Discontinuous Conduction Mode (DCM) in accordance with a determination of a time when a voltage between a first stage and a second stage of the switching transistor is resonated. In accordance with the present invention, the SMPS of a quasi-resonant switching mode can be driven with high efficiency regardless of a signal level of an input voltage and the amount of an output terminal load.