Cello‐oligosaccharides are biologically important molecules that can elicit a defensive immune response in plants and improve the health of animals. Cellulose, a polymer of glucose linked by β‐1,4‐glycosidic bonds, is an ideal feedstock for synthesis of cello‐oligosaccharides. However, cello‐oligosaccharides rapidly degrade under the conditions used for cellulose hydrolysis. Here, cellulose was hydrolyzed over a carbon catalyst in a semi‐flow reactor to achieve a high yield of cello‐oligosaccharides (72 %). The excellent activity of the oxidized carbon catalyst, the adsorption of cellulose on the catalyst, and the high space velocity of products in the reactor were essential. Moreover, a method for quantification of individual cello‐oligosaccharides was developed, which suggested a reduction in the rate of hydrolysis with a reduction in chain length.