The present invention relates to the method for tagging of carbohydrates with active methylene compound. Particularly, it relates to the method for tagging of carbohydrates with active methylene compound comprising the step of preparing carbohydrate conjugate in which carbohydrate and methylene compound are combined by mixing carbohydrate mixture and methylene compound under aqueous polar aprotic solvent containing amine base catalyst. The tagging method of the present invention does not need many kinds of chemical reagent and the reactions can be taken even in the presence of certain amount of impurities. So, it can be used for the analysis of oligosaccharide present in the various kinds of samples.