The effects of dioctyl phthalate and inorganic filler, mica, on the sound insulation property of poly(vinyl chloride) (PVC) were investigated in this work. The stiffness and mass laws, which are the common theoretic tools to predict the soundproof properties of materials, were used to analyze the sound transmission loss (STL). The experimental results revealed that the stiffness and mass laws can describe well the sound insulation property of PVC/mica composites. The stiffness and surface density are important factors influencing the improve-ment of STL. With the increase of content of mica, STL and resonance frequency, f mn , of PVC/mica composites increase. Moreover, the change of STL in the stiffnesscontrolled region is more obvious than that in the masscontrolled region, because the addition of mica in PVC leads to a greater increase in the stiffness.