The present invention relates to a packet data transmission method in an OFDMA system and an apparatus thereof. The present invention provides a user packet data transmission method including generating a RACH burst for channel information estimation, selecting a first antenna among a plurality of antennas, transmitting the RACH burst through the first antenna to a base station, receiving a data transmission acceptance signal from the base station, in response to the RACH burst, transmitting user packet data through the first antenna when receiving a signal indicating that the data transmission is allowed, and selecting a second antenna that is difference from the first antenna and transmitting a RACH burst through the second antenna when receiving a signal indicating that the data transmission is not allowed, and a transmitting apparatus of a mobile station having the plurality of antennas. According to the present invention, the RACH burst and user packet data are transmitted by using a mobile station having a plurality of antennas in an LTE-applied OFDMA system, and therefore data transmission can be performed with a wide bandwidth and high data rate and a deep fading period that may occur during the data transmission can be reduced, thereby preventing call dropping and improving data transmission performance.