Random access method using NDI and user equipment for the same

A method of a user equipment (UE) communicating with a base station (BS) in a mobile communication system, the method comprising:

• transmitting uplink (UL) data according to a first Hybrid Automatic Repeat reQuest (HARQ) process;
• setting a New Data Indicator (NDI) value corresponding to the first HARQ process to a first value;
• and
• receiving a downlink (DL) message including an NDI having a second value, the NDI having the second value used by the UE to determine whether to retransmit the UL data in a first state in which a random access procedure is not performed and to determine whether to retransmit a random access preamble in a second state in which the random access procedure is performed,
• wherein the UE in the first state determines to not retransmit the UL data when the first value is different from the second value and determines to retransmit the UL data when the first value is the same as the second value, and
• wherein the UE in the second state determines to retransmit the random access preamble when the first value is different from the second value and determines to not retransmit the random access preamble when the first value is the same as the second value.