A data transmission method and an apparatus to communicate data on multiple carriers in the mobile communication system are provided. A random access method of a terminal in a mobile communication system including primary and secondary cells operating on multiple carriers according to the present invention includes communicating data after random access in the primary cell, receiving, when the random access is triggered in the secondary cell, information for use in the secondary cell random access from the primary cell, transmitting a preamble in the secondary cell based on the received information, monitoring the primary cell to receive a Random Access Response for the secondary cell, and applying, when the Random Access Response for the secondary cell is received, the information carried in the Random Access Response to the secondary cell in which the preamble has been transmitted.