The Jarman-Bell Principle among ungulates explains that larger species feeding on high-fiber foods have developed fermentation chamber such as rumen than smaller species feeding on low-fiber foods. Among arboreal squirrels, it seems that larger Petaurista species feed on high-fiber leaves and smaller species feed on low-fiber fruits and seeds. Therefore, relative length of the cecum may be positively correlated with body mass which is associated with tendency of folivory. To test this hypothesis, we examined body mass, head-body length, and the alimentary tract lengths (small intestine, cecum, colon, large intestine, and total intestine) of 13 arboreal squirrel species (n = 19) from six genera. Spearman rank correlation analysis showed that larger folivorous Petaurista had a longer fermentation chamber. Smaller granivorous squirrels might have longer small intestine. These differences in alimentary tract morphology of arboreal squirrels could reflect feeding adaptation.