Annealing effects on molecular orientation and multilayer order in vacuum-deposited permethyldecasilane thin films were investigated by means of X-ray diffractometry (XRD) and UV absorption spectroscopy. An annealing-induced multilayered structure with the molecular orientation oblique to the substrate was observed by XRD. Strong incident-angle dependence of the absorption spectrum of the as-deposited film was mainly due to a spectral component attributed to a molecular orientation normal to the substrate. After annealing, the absorption spectrum and its incident angle dependence significantly changed, which is explained in terms of molecular reorientation. Detailed spectral analysis to assign each component to specific molecular orientation is presented.