Due to the capacitor being a critical component that is prone to failure in switch mode power supplies (SMPS), it is critical to identify and monitor the electrical parameters, namely the equivalent series resistance (ESR) and capacitance (C). This research proposes an online identification approach for capacitor ESR and C for continuous conduction mode (CCM) and discontinuous conduction mode (DCM) DC-DC buck converters. The calculation model is developed from the ripple component of capacitor voltage. By sampling the ripple component of capacitor voltage at the switch on or switch off moment, and capturing the first zero crossing time of the ripple component of capacitor voltage. This identification method does not use any current sensor and is effective for CCM and DCM buck converters working under different circuit parameters. The implementation of the proposed identification method is provided, and the details of implementation are provided. The simulation results verify the proposed identification method is effeteness.