© 2019 Elsevier Ltd Paramylon is a β-1,3-glucan photosynthesized by Euglena. In this study, the four paramylon ester derivatives, paramylon propionate (PaPr), paramylon butyrate (PaBu), paramylon valerate (PaVa), paramylon hexanoate (PaHe), were prepared and their crystal structures were determined by wide-angle X-ray diffraction. All of the reflections are indexed to a hexagonal crystal system and all of the molecular chains have five-fold screw symmetry along the molecular axes, despite of the different side-chain lengths of the ester groups. The crystal lattice parameters increase with increasing of side-chain length of the ester groups. The crystalline elastic modulus of PaPr, PaBu, and PaVa parallel to the fiber axis (E l ) were determined by time-resolved X-ray diffraction at SPring-8, which is a large synchrotron radiation facility. The crystalline elastic modulus of PaPr, PaBu, and PaVa are 2.5, 1.9, and 1.0 GPa, respectively. The decreasing E l values are mainly caused by increasing of cross-sectional area of the molecular chains, which is affected by the side-chain length. The E l values of the paramylon esters are one-tenth those of previously reported cellulose esters. This seems to be because of the different helical structure of the backbone chain.