As a dielectric composite material which can serve as a candidate film capacitor, a polyvinylidene fluoride (PVDF)-based composite film embedded with n-dodecylammonium-modified montmorillonite (12CH₃-MMT) was investigated. 12CH₃-MMT was first synthesized by an ion exchange method, and it was delaminated into a single to several ten layers by stirring in a PVDF/N,N-dimethylformamide solution at 65 °C for 4 days. The resultant 12CH₃-MMT/PVDF composite film exhibited higher breakdown strength (E_bd) than a pure PVDF film, and E_bd increased as the amount of 12CH₃-MMT in the composite film increased up to 10.0 wt %, which was consistent with the prediction based on a simplified physical simulation. The increase in the amount of 12CH₃-MMT more than 10.0 wt % lead to the decrease in E_bd contrary to the prediction, which could be attributed to the mutual contact of 12CH₃-MMT fillers in the composite by TEM observation. It was found that 12CH₃-MMT were homogeneously dispersed in PVDF with a wide range of amount up to 10.0 wt %, and that the 12CH₃-MMT/PVDF composite film with higher energy density than a biaxially oriented polypropylene were obtained.