The main carbohydrate component of grey oyster mushroom (Pleurotus sajorcaju (Fr.) Sing.) is β- glucan, a polysaccharide of glucose linked by β-(1-3), β-(1-6) and gif.latex?\alpha-(1-3) glycosidic bonds. The β-glucan in the form of oligosaccharides has been recognized as an adjuvant and exhibits prebiotic properties. In this report, oligosaccharides were extracted from a fine powder of grey oyster mushroom by a combination of microwave radiation heating and acid hydrolysis. The use of microwave radiation provides more heating efficiency and shortens reaction time. The hydrolysis product was purified by precipitation with propanol. The effects of reaction temperature (70 to 150 °C), a reaction time of 15 min, and HCl concentration (0.2, 0.4, 0.6, and 0.8 M) on the degree of polymerization (DP) of the extracted saccharides were determined. The extracted saccharides were characterized by using size exclusion chromatography and NMR. The analysis showed smaller sizes of extracted saccharides (DP = 2 to 5) when using the higher HCl concentration of 0.6 M. The maximum yield (2.24%) of extracted oligosaccharides was obtained at the conditions of 120 °C, 15 min with 0.6 M HCl as a catalyst.