The present invention relates to a process for improving pegylation reaction yield of r-metHuG-CSF comprising conjugating r-metHuG-CSF to a PEG aldehyde at a free amine moiety at the N terminal end on the G-CSF in presence of a reducing agent in a pegylation buffer solution comprising a polyol having the formula CnH2n+2On where n is from 3 to 6, or a carbohydrate, or a derivative thereof wherein the concentration of said polyol or carbohydrate or derivative thereof is in the range of 0.1% to 10% w/w.