A method for transmitting wireless power in a wireless charging system and a wireless power transmitting unit (PTU) and provided. The method for transmitting wireless power in a wireless charging system includes receiving information related to a voltage from each of a plurality of power receiving units (PRUs); identifying a voltage ratio of each of the plurality of PRUs based on the received information, wherein the voltage ratio is a current voltage relative to a first voltage; determining a PRU among the plurality of PRUs based on the identified voltage ratio; and adjusting transmission power according to a voltage setting value of the determined PRU.