Inulin-type oligosaccharides with different DP were prepared by size-exclusion chromatography and purity of each oligosaccharide was determined by HPLC equipped with cyclodextrin-bond column. The purities of obtained inulin-type oligosaccharides with different DP were more than 98% by one-step process. The DP and molecular weight were obtained through ESI-MS in negative mode. The characterization of the inulin-type oligosaccharides with different DP was studied by MS/MS spectra obtained by collision-induced dissociation of molecular ions ([M-H](-)). When the DP was lower, the fragment ions were formed through cross-ring cleavages of two bonds within the sugar ring and glycosidic cleavages. However, with the increase of DP, the ions resulting from glycosidic cleavages between two sugar residues were predominant.