A method of performing a random access channel (RACH) procedure by a user equipment (UE) in a wireless communication system is disclosed. The method includes transmitting a physical random access channel (PRACH) and a physical uplink shared channel (PUSCH) via a message A, and receiving a message B related to contention resolution in response to the message A. An index related to a reference signal used for transmission power allocation to the PUSCH is same as an index of a resource used for channel measurement for transmission of the PRACH, based on that the PRACH and the PUSCH are transmitted via the message A.