Polymer-clay nanocomposites are well-known high-performance materials with a superior tensile modulus. However, in the case of composites with polyimide (PI), additional functions require study because PI is a high-performance material in itself. Significant enhancement of thermal conductivity, which is closely related to the state of clay dispersion, is expected for a polymer-clay nanocomposite. In this study, variations in the thermal diffusivity of PI-clay nanocomposite films prepared by different methods were investigated. The thermal diffusivity of PI-clay nanocomposite film increased at low clay content only when unmodified clay was used, where the clay morphology was a layered structure dispersed on a nanometer scale. Moreover, the thermal diffusivity could be enhanced by controlling the tensile stress induced by spontaneous shrinkage of the film during thermal imidization. These results demonstrated that the thermal diffusivity of PI-clay nanocomposite films is significantly affected by the dispersion and/or arrangement states of the clay. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 119: 3010-3018, 2011