The present invention provides a method performed by a secure digital (SD) card supporting both an SD mode and a peripheral component interconnect express (PCIe) mode for initializing the SD card. The method includes: (a) after receiving a first supply voltage through a first voltage supply pin from a host coupled to the SD card, entering the SD mode if the SD card is not in the PCIe mode and a CMD0 command for entering the SD mode is received through a command pin from the host coupled to the SD card; and (b) after receiving the first supply voltage through the first voltage supply pin from the host coupled to the SD card, performing a PCIe linkup process if the SD card is not in the SD mode and a second supply voltage is received through a second voltage supply pin from the host coupled to the SD card. The SD card enters the PCIe mode if the PCIe linkup process succeeds.