Elastic moduli of the crystalline regions of cellulose triesters in the direction both parallel E(l) and perpendicular E(t) to the chain axis are measured by x-ray diffraction. E(l) values for cellulose triesters are as follows : cellulose triacetate (CTA), 33.2 GPa; cellulose tripropionate (CTP), 21.6 GPa; cellulose tributyrate (CTB), 17.6 GPa; cellulose trivalerate (CTV), 17.9 GPa. These are very small compared with that of cellulose I (134 GPa) and resemble each other despite the fact that the skeletal structure of CTP (3(2) helix) is much different from those of other cellulose triesters (2-fold helix). The small E(l) values for cellulose triesters can be explained by both the large cross-sectional area of one molecule in the crystal lattice and the lack in intramolecular hydrogen bonds. E(t) values for cellulose triesters are as follows : CTA (310), 2.8 GPa; CTP (110), 1.4 GPa; (020), 2.1 GPa; (120), 2.2 GPa; CTB (110), 1.1 GPa. These are one of the smallest E(t) values already reported, and decrease with increasing the length of the side chain. Intermolecular interaction is considered to decrease with the length of acyl groups. The large E(t) value in the b direction of CTP reflects the dense packing of the chains in the crystal lattice.