A method for manufacturing a metal gate stack structure in gate-first process comprises the following steps after making conventional LOCOS and STI isolations: growing an untra-thin interface layer of oxide or oxynitride on a semiconductor substrate by rapid thermal oxidation or chemical process; depositing a high dielectric constant (K) gate dielectric on the untra-thin interface oxide layer and then performing rapid thermal annealing; depositing a TiN metal gate; depositing a barrier layer of AlN or TaN; depositing a poly-silicon film and a hard mask, and performing photo-lithography and the etching of the hard mask; after photo-resist removing, etching the poly-silicon film/metal gate/high-K gate dielectric sequentially to form the metal gate stack structure. The manufacturing method of the present invention is suitable for integration of high-K dielectric/metal gate in nano-scale CMOS devices, and removes obstacles of implementing high-K/metal gate integration.