Disclosed is an apparatus and method for transmitting data in a wireless communication system. The apparatus for transmitting a packet data in a wireless communication system includes a Media Access Control (MAC) layer for determining the number of preambles to be inserted based on a feed-back reply signal from a receiver, and generating a multi-preamble aggregation packet by inserting the determined number of preambles into packets received from an upper layer and a physical layer for forming the multi-preamble aggregation packet generated in a data processing unit, by using a physical layer packet, and transmitting the formed multi-preamble aggregation packet.