New highly effective promoter system for the glycosylation of thioglycoside, the combined use of stoichiometric amount of either N-iodosuccinimide or N-bromosuccinimide and a catalytic amount of TrB(C6F5)(4), was developed and was successfully applied to the one-pot sequential synthesis of trisaccharides. In the first glycosylation step, 3,3,6-tri-O-benzyl-2-O-p-toluoyl-beta-D-glucopyranosyl phenylcarbonate was treated with thioglycosides in the presence of a catalytic amount of TrB(C6F5)(4) alone, and the following glycosylation with methyl alpha-D-glycosides was carried out by further addition of either N-iodosuccinimide or N-bromosuccinimide to the initially resulted reaction mixture to afford the corresponding trisaccharides in one-pot manner.