A method is provided for performing a hybrid automatic repeat request (HARQ) by a user equipment (UE). An uplink (UL) grant for a first subframe of a second serving cell is received through a first serving cell. UL data in the first subframe of the second serving cell is transmitted according to the UL grant. When a not-acknowledgement (NACK) for the UL data is received through a physical HARQ indicator channel (PHICH) in a subframe i of the first serving cell, the UE performs a non-adaptive retransmission for the UL data in a second subframe of the second serving cell. When a PHICH resource corresponding to the UL data is not present in the subframe i, the UE does not perform the non-adaptive retransmission in the second subframe of the second serving cell.