Phlogopite with layered silicate structure had been firstly chemically modified via an in situ intercalation method, and phlogopite-polymer nanocomposite films were prepared from 2,2'-bis (3,4-dicarboxyphenyl) hexafluropropane dianhydride (6FDA) and oxydimethyl aniline (ODA) in N,N-dimethylacetamide as a solvent by using in-situ polymerization process combined with ultrasonic dispersion and multi-step curing. The structure of phlogopite minerals and its polymer nanocomposites were characterized by X-ray diffraction (XRD) and infrared spectra (FTIR) respectively. The experimental results indicated that the phlogopites with layered nanostructure had lost their ordered structure and had been exfoliated or intercalated. Thereafter, they were dispersed randomly in the polyimide matrix. The dependence of dielectric properties and thermal stabilities of the nanocomposite films on the phlogopite content and frequency were studied.