The present disclosure relates to a method and a device for transmitting data in a wireless power transmission system. The method for transmitting data by a wireless power transmitter in a wireless power transmission system may comprise the steps of: receiving from a wireless power receiver, within a first control error interval, a first control error packet containing a control error value with respect to power transmitted from the wireless power transmitter; determining the size of a first data packet on the basis of the length of the first control error interval; and transmitting the first data packet to the wireless power receiver.