The effective thermal conductivity (ETC) of graphite flake (GF)/Al composites is significantly influenced by the anisotropic thermal conductivity of GFs and the interfacial thermal resistance between both components. A two-dimensional (2D) image-based simulation was used in this study to investigate the effect of both the orientation of GFs and interfacial thermal resistance on the ETC of the composite. 10 vol% GF/Al and 20 vol% GF/Al composites were fabricated via spark plasma sintering. The microstructure ETC, and the relative density of the GF/Al composites were determined. The experimental ETCs were smaller than calculated ETCs using the rule of mixture. Additionally, the calculated ETC exhibited a decrease of 9.9% due to the effect of GF anisotropic thermal conductivity, and the ETC values decreased by 14.2% due to the effect of the interfacial thermal resistance at the Al–GF interface of the samples, which was determined to be in the range of 5.62–6.41 × 10⁻⁸ m² K W⁻¹.