A method and apparatus are disclosed from the perspective of an User Equipment (UE). In one embodiment, the method includes the UE deriving a first pathloss value from a first pathloss reference of a serving cell, wherein the first pathloss value is used for deriving a power headroom value included in a first power headroom report. The method also includes the UE deriving a second pathloss value from a second pathloss reference of the serving cell after deriving the first pathloss value, wherein the second pathloss reference is used for power control for a first Physical Uplink Shared Channel (PUSCH) transmission on the serving cell. The method further includes the UE deriving the pathloss change based on the first pathloss value and the second pathloss value. In addition, the method includes the UE determining whether a second power headroom report is triggered based on whether the pathloss change is more than a threshold.