A theoretical treatment for the effect of thermal vibration on the X-ray scattering intensity for a helical polymer crystal is presented. The treatment involves assumptions that (1) the unit cell of the crystal contains only one polymer chain and (2) fluctuations of the atomic positions are small. The temperature factor of the intensity of Bragg reflections is expressed in terms of mean squares of fluctuations of helical parameters. The diffuse scattering is also considered in a special case.