Method of handling hybrid automatic repeat request feedback and related communication device

A method of handling a hybrid automatic repeat request (HARQ) feedback in a subframe for a first communication device in a wireless communication system according to a first uplink (UL)/downlink (DL) configuration and a second UL/DL configuration, the method comprising:

• determining whether the HARQ feedback exists in a physical HARQ indicator channel (PHICH) in a DL control region of the subframe transmitted by a network of the wireless communication system according to the first UL/DL configuration of the first communication device and the second UL/DL configuration of a second communication device in the wireless communication system communicating with the network, wherein the first UL/DL configuration and the second UL/DL configuration are different;
• detecting the HARQ feedback in the PHICH of the subframe and detecting the HARQ feedback according to DL control information (DCI) in a physical DL control channel (PDCCH) in the DL control region, if the HARQ feedback exists in the PHICH;
• and
• detecting the HARQ feedback according to the DCI in the PDCCH in the DL control region, if the HARQ feedback does not exist in the PHICH.