A method for controlling transmission power of shared control channel for high speed downlink shared channel (HS-SCCH) in an UMTS system adopting a high speed downlink packet access (HSDPA) is disclosed in which a power offset value for power control of HS-SCCH is determined by a serving radio network controller and transmitted as a control frame or message type to a base station. A first PO field for transmitting a PO value for non-handover or soft handover of a dedicated physical control channel, a second PO field for transmitting a PO value used for the primary cell and a third PO field for transmitting a PO value used for the non-primary cell are selectively implemented in the control frame or message type, so that the transmission power of HS-SCCH can be suitably controlled according to changes in environment or circumstances of the radio link.