The work aims to study the influence of content and particle size of Gd2Zr2O7 on the thermal conductivity of (Sm2Ce2O7)1-x(Gd2Zr2O7)x composite materials. Two series of (Sm2Ce2O7)1-x(Gd2Zr2O7)x composite materials were prepared by nano and micron powders. The phase-structure of composite materials was identified by X-ray diffraction method. The micro-structure of composite materials was observed by scanning electron microscope. The specific heat capacity of composite materials was computed by Neumann-Kopp rule, and the thermal diffusivity of composite materials was measured by laser-flash method. Finally, the thermal conductivity of composite materials was obtained in light of the density, specific heat capacity and thermal diffusivity. The influence of particle size and content of Gd2Zr2O7 on thermal conductivity of composite materials was analyzed according to the final thermal conductivity results. The synthesized powders show single defect-fluorite structure. The largest specific surface area of nano-scale Gd2Zr2O7 is about 15.413 m2/g and the minimum specific surface area for micro-scale Sm2Ce2O7 is about 0.226 m2/g. These two series of (Sm2Ce2O7)1-x(Gd2Zr2O7)x composite materials also exhibit single fluorite crystal-lattice, while the grain size is not uniform. When x=0.5, the typical nano-scale grains are found in the composite materials prepared by nano-powder. The addition of micron-scale Gd2Zr2O7 does not influence the phonon-thermal conductivity obviously, but inhibits the photon thermal conductivity. The introduction of nano-scale Gd2Zr2O7 can reduce the phonon thermal conductivity of composite materials, while cannot inhibit the photon thermal conductivity obviously. The thermal conductivity of two series of composite materials is lower than YSZ.