Disclosed are a method and apparatus for transmitting downlink data in a wireless communication system. More specifically, a method for transmitting downlink data to a user equipment (UE) in a wireless communication system comprises receiving, by a Mobile Management Entity (MME), a Downlink Data Notification message from a Serving Gateway (S-GW); and in case the MME detects that a UE is in a sleep mode, transmitting, by the MME, a response message for indicating buffering of downlink data to the S-GW, wherein the response message includes buffering time of the downlink data, and the buffering time is a time period until the UE is expected to switch from the sleep mode to a reachable state.