Extraction of mannan oligosaccharides from the yeast cell wall and methods of deproteinization were studied. We extracted crude mannan oligosaccharides by the dilute alkali- Sevage method. The percentages of deproteinization and mannan oligosaccharide loss were compared as indexes using the Sevage method, trichloroacetic acid method, and hydrochloric acid method. The results showed that the hydrochloric acid method exhibited the highest percentage of deproteinization, but only a little higher percentage of mannan oligosaccharide loss than the other two methods.