Abstract: Maltodextrin was derivatized with 1-(4-isopropyl) phenyl-3-methyl-5-pyrazolone (PPMP) in aqueous ammonia and derivatization products were analyzed by RP-HPLC separation and ESI-MS identification. The results indicated that mono-PPMP derivatives are still principal product for oligosaccharides. The mono-PPMP-derivatives for oligosaccharides from maltodextrins show satisfactory response to ESI-MS both in positive-ion mode and in negative-ion mode. The mixture of oligosaccharides derivatives with degree of polymerization 1—17 was successfully separated by RP-HPLC. A new method for detecting the composition of oligosaccharides mixture with PPMP pre-column derivatization by HPLC coupled with on-line ESI-MS techniques was established. The method shows following merits: simple treatment for sample, fast analysis and accurate qualitation, and it can be used as a reference method for research of oligosaccharides’ quality control and structure function relationship.