This invention relates to a method for performing Power Headroom reporting procedure, adapted for a user equipment, wherein the user equipment is operated in a plurality of activated serving cells, the method includes the steps of: providing a prohibitPHR-Timer; obtaining a plurality of downlink path loss values respectively corresponding to the activated serving cells; triggering a power headroom report when the prohibitPHR-Timer expires or has expired and at least one of the downlink path loss values has changed more than a PathlossChange threshold since the last transmission of a PHR; and reporting the power headroom report.