The thermal expansion behavior of hydrate paramylon between 100 and 300K has been investigated using synchrotron X-ray powder diffraction. The X-ray diffraction profile at 300K showed a typical pattern of the hydrate triple helical (1→3)-β-d-glucan with a hexagonal unit cell (a=15.782Å and c=18.580Å). On cooling, the hydrate paramylon had converted to a "low-temperature phase" around 270K. On passing through the phase transition, the a-axis and c-axis values decreased and increased, respectively, and the low-temperature phase at 100K exhibited a hexagonal unit cell (a=15.586Å and c=18.619Å). The phase transition took place reversibly. Below the transition point, both the a-axis and c-axis values decreased linearly. The thermal expansion coefficients are: α(a)=1.50×10(-5)K(-1), α(c)=0.33×10(-5)K(-1), and β=3.08×10(-5)K(-1).