For developing thermally stable polymer films exhibiting high thermal conductivity, fluorinated polyimides (PIs) were hybridized via 'in situ hybridization method' and 'direct mixing method' with magnesium oxide (MgO) nanoparticles possessing high thermal conductivity and good insulating property. In the former method, MgO nanoparticles were spontaneously generated and precipitated in PI films by thermal decomposition of a soluble precursor, magnesium acetate tetrahydrate (MgAc). The Far-IR absorption spectra, WAXD patterns, and solid-state ²⁵Mg NMR spectra of PI/MgO hybrid films demonstrated that MgAc was thermally decomposed to MgO nanoparticles during curing up to 380 °C. In addition, the cross-sectional SEM images of the hybrid film showed that spherical MgO nanoparticles (average diameter : ∼50 nm) were homogeneously dispersed without aggregation in the hybrid films. The thermal diffusivities along the out-of-direction (α_⊥) of the hybrid films were measured using the temperature wave analysis, and their thermal conductivities (λ) were estimated from α_⊥, specific heats, and densities. For the hybrid films prepared by both the methods, the λ values are proportionally increased as the MgO content increase, and the λvalues are in good accord with the predicted values based on the Bruggeman's theory.