Conventionally, boost converter is used for high step-up energy harvesting system, but it is needed to be operated in discontinuous conduction mode (DCM) because of the limitation of the topology. Therefore, it causes more conduction loss and current stress on the switch. In this thesis, the control IC for boost-flyback converter in energy harvesting applications is proposed. It can greatly increase the voltage conversion ratio with the cascode of boost converter and flyback converter. In addition, this topology can recycle the energy of the leakage inductance produced from coupled inductor to load. Thus, the efficiency can be increased and the stress on the main switch can be reduced. Finally, The boost-flyback converter with input voltage of 0.2-0.3 V, output voltage of 5 V and output power of 20 mW is implemented to verify the feasibility of the proposed controller.