Poly(vinyl octanal acetal) (PVO) was prepared from poly(vinyl alcohol) (PVA) in which the main chain had three different tacticities and the stereoregularity was characterized by H-1 NMR spectroscopy. a-PVO prepared from atactic-PVA and s-PVO from syndiotactic-PVA had similar fractions of cis- and trans-acetals in the main chain, but i-PVO from isotactic-PVA had different ones. Ultrathin films of these polymers were prepared by the Langmuir-Blodgett (LB) technique, and the thermal relaxation of their multilayer structure was investigated by the energy-transfer method. When the LB films were composed of polymers which had a main chain with a similar stereostructure, the polymers were intermingled with each other by the thermal treatment and the onset of the diffusion of polymer segments was affected by the glass-transition temperature of the bulk. In contrast, the polymers with different stereostructures in each layer were not mixed with each other, but phase-separated, presumably forming droplets in the ultrathin films. It is concluded that the relaxation behavior of polymer LB films strongly reflects the stereoregularity of the sample polymers.