1. A method for dynamically changing the priority of a thread in a simultaneous multithread (SMT) processor comprising the steps of: generating a first thread priority no operation (NOP) instruction to change an instruction execution priority of a selected thread from said first and second threads; decoding said first thread priority NOP instruction; and writing a special code for modifying said instruction execution priority of said selected thread into a group completion table (GCT) in response to a dispatch of said first thread priority NOP instruction.