Thermally induced phase separation was studied by the light scattering in polypropylene/methyl salicylate system. Data could be well fitted with the linear Cahn theory for spinodal decomposition (SD) in the early stage of phase separation. Characteristic properties of the early stage of SD, such as an apparent diffusion coefficient and an interphase periodic distance, were obtained. The periodic distance ranged from 3 m to 4 m. The growth of the phase-separated structure obeyed power-law scaling in the later stage, and the structure factor could be scaled into a universal time-independent form. Domain sizes obtained from the light-scattering measurements were consistent with the optical microscope measurements.