A method of transmitting a signal by a wireless power transmitter in a wireless charging system, wireless power transmitter, and a wireless power receiver are provided. The method includes transmitting a first signal; transmitting a second signal; detecting a load change during a period in which the second signal is transmitted; and extending a transmission period of the second signal based on the detected load change. The wireless power transmitter includes a power transmission unit configured to transmit a first signal and a second signal; a sensing unit configured to detect a load change during a period in which the second signal is transmitted; and a controller configured to extend a transmission period of the second signal based on the detected load change.