A method of transmitting data packets in a mobile communication system using at least one relay station (RS) and using at least two frequency carriers is disclosed. More specifically, the method comprises transmitting at least one subpacket of a first data packet to a mobile station (MS) on a first frequency carrier, and transmitting at least one subpacket of a second data packet to the MS via the at least one RS. Here, the at least one subpacket of second data packet from the at least one RS is transmitted on a second frequency carrier, a first subpacket of the first data packet and a first subpacket of the second data packet is transmitted via a main channel of the first frequency carrier and the main channel of the second frequency carrier, respectively, and at least one subsequent subpacket of the first data packet and at least one subsequent subpacket of the second data packet are transmitted via at least one assistant channel of the first frequency carrier and the at least one assistant channel of the second frequency carrier, respectively.