Graphene paper is a potential candidate for heat spreader because of its special thermal conductivity. As graphene paper is assembled by the graphene sheets, the size of graphene sheets has an important effect on assembly method, microstructure and thermal conductivity of the graphene paper. The influence of the size of graphene sheets on the microstructure and thermal property of graphene paper was studied. Firstly, the filtration-assembling method was used to fabricate graphene oxide paper. Then, the grapheme paper was obtained from the heat-reduced process of the graphene oxide paper under the Ar/H₂ atmosphere. The SEM(scanning electron microscope) characterization result shows that the graphene paper with the bigger graphene sheet size is of higher density. The XRD(X-ray diffraction) characterization result also shows that the crystalline of the graphene paper is increased when the graphene sheet size is bigger. When graphene sheet size increased from the range of 0.3 μm～5 μm to that of 50 μm～100 μm, the thermal conductivity is about 632.8 W/mK and 683.7 W/mK respectively，which is increased by 8%. 