A method and apparatus for synchronizing time in a Gigabit Passive Optical Network (GPON) in which an Optical Line Terminal (OLT) sends a downstream frame including a Physical Control Block downstream (PCBd) field and a payload field to an Optical Network Unit (ONU); the OLT determines a time value associated with a certain bit in the PCBd field of the downstream frame; and the OLT sends an Operation, Administration and Maintenance (OAM) message defined by a GPON protocol carrying the time value to the ONU, wherein the ONU uses the time value to adjust a local time of the ONU.