Several trisaccharides were prepared by efficient highly astereoselective one-pot sequential glycosylation using glucosyl p-trifluoromethylbenzylthio-p-trifluoromethylphenyl formimidates (abbreviated to: glucosyl thioformimidates) in the presence of a catalytic amount of TfOH. Factors that controlled the high astereoselectivity were determined by characteristic properties of thioformimidate groups contained both in glucosyl donor and acceptor.