Abstract: Dynamic speckles appear when a cylinder rotating with a constant angular velocity is irradiated by a Gaussian light beam. The statistical property of dynamic speckles in far-field diffraction is acquired when the irradiated area is larger than the cylinder surface. The normalized space-time cross-correlation of dynamic speckle intensity fluctuations is obtained with different cylinder radii in different incidence conditions. The results show that the correlation is improved when the cylinder radius and rotating speed decrease. When the cylinder rotates at a constant speed, the peak value of cross-correlation function descends gradually as the distance between the two observation points increases, whereas the peak position increases. In the case of approximate point irradiation, the spatial correlation length is almost unchanged and the correlation time extends obviously as the cylinder radius increases.