A method and apparatus estimate a delay of buffer data of a User Equipment (UE) in a mobile communication system. A message comprising the size of buffer data is received from a UE. The size of the buffer data of the UE is distributed into a plurality of groups based on a time point of receiving the message comprising the size of the buffer data of the UE. A delay for each of the plurality of groups is estimated using a time interval of execution of delay estimation for the buffer data of the UE.