HARQ feedback channel design for carrier aggregation (CA) is proposed in a multi-carrier LTE/LTE-A system. In one novel aspect, a predefined rule for HARQ feedback channel format switching is adopted by the system. Different HARQ formats are supported: single component carrier (non-CA) mode, carrier aggregation with small payload size (CA-S) mode, carrier aggregation with large payload size (CA-L) mode, and fallback mode. From the various CA and non-CA formats, the format to be used for uplink HARQ feedback channel is determined based on: UE capability for the maximum number of CCs supported; CC configuration information by RRC layer; and detection results of downlink schedulers. The CC configuration information may include the number of CCs that are configured and a specific HARQ format to be used. Because more reliable upper layer configuration is used to make the HARQ format-switching decision, the risk of UE and eNB mismatch is greatly reduced.