Objective To establish a precolumn derivation RP-HPLC method for the determination of the monosaccharide compositions in Cydonia oblonga Mill. polysaccharides, and to test their inhibitory activities of Cydonia oblonga Mill. polysaccharides on protein tyrosine phosphatase 1B (PTP1B). Methods The polysaccharides were extracted by hot dis tilled water, and precipitated by alcohol. An in vitro screening model was applied to test the PTP1B inhibitory activities. The polysaccharides were derived with 1-phenyl-3-methyl-5-pyrazolone(PMP)after hydrolyzed with 2 mol·L~(-1) H_2SO_4 and trifluoroacetic acid(TFA), and then the PMP derivates of monosaccharides were separated by RP-HPLC. Results Cydonia oblonga Mill. polysaccharides had the monosaccharide compositions of mannose, rhamnose, glucose, galactose, and arabinose with the molar ratio of 0.10, 0.72, 1.12, 4.23, and 4.73, respectively. Moreover, Cydonia oblonga Mill. polysaccharides showed certain PTP1B inhibitory activity with IC_(50) value of 2.068 mug·mL~(-1). Conclusion The method is simple, sensitive, accurate and reliable.It can be applied for the analysis of monosaccharide compositions and the quality control of Cydonia oblonga Mill. polysaccharides.