A method and apparatus are disclosed. In an example from the perspective of a User Equipment (UE), the UE receives an uplink (UL) grant. The UL grant is indicative of a first Physical Uplink Shared Channel (PUSCH) transmission on a first Transmission/Reception Point (TRP) of a first cell. The UL grant is not indicative of a PUSCH transmission on a second TRP of the first cell. The UE transmits a Power Headroom Reporting (PHR) Medium Access Control (MAC) Control Element (CE). Based on the UL grant, the PHR MAC CE is indicative of a first power headroom (PH), associated with the first TRP, based on a real PUSCH transmission, and is indicative of a second PH, associated with the second TRP, based on a reference PUSCH transmission.