The glycosyl onium ions are interesting species in carbohydrate chemistry. For instance, the well-known solvent effect in glycosylation reaction is explained by glycosyl onium ion intermediates. But, the investigation examples of characterization and reactivity of unstable glycosyl onium ions are few. We generated glycosyl sulfonium ions form highly reactive glycosyl triflate, generated by electrochemical oxidation method quantatively. The both alfa- and beta- glycosyl sulfonium ions were characterized by NMR and cold-spray MS. Furthermore, it is revealed that the alfa-glycosyl sulfonium ion has higher reactivity than beta-glycosyl sulfonium ion by time-course NMR analysis. From the stereochemical outcome observed in the glycosylation of glycosyl sulfonium ion with MeOH indicates that a simple SN2 attack on the glycosyl sulfide can be excluded as a reaction pathway.