For the samples of Chinese juniper (Sabina Przewalskii) and Japanese cedar (Cryptomeria japonica), we compared the carbon isotope, carbon content and chemical composition of crude cellulose prepared by diglyme-HCl method with those of alpha-cellulose Our results suggest that the diglyme-HCl method, which appears to be a simple, fast method and safe method for isolating cellulose from tree-ring samples, is effective for isotope analysis of large-number and small samples.