A method of power headroom reporting (PHR) is proposed. A UE is configured with a plurality of component carriers (CCs) and is served by one or more power amplifiers (PAs) in a wireless system with carrier aggregation. The UE determines transmit power limitation (TPL) information that comprises a set of TPL values, each TPL value corresponds to a UE-configured maximum transmit power for UE-level, PA-level, and CC-level. The TPL information is then reduced to non-redundant TPL values. Based on the non-redundant TPL values, the UE determines power headroom (PH) information that comprises a set of PH values. Each PH value equals to a TPL value subtracted by a UE-calculated transmit power. The UE reports the PH information to a base station via a fixed-length or variable-length MAC CE at each PHR reporting instance.