Graphite flakes reinforced Al matrix composites (G_f /Al) with low density, good machining property and high thermal conductivity are considered an excellent heat sink materials used in electronic industry. When the composites are manufactured by liquid method such as liquid infiltration, it is easy to achieve a high thermal conductivity composite. However, the Al₄C₃ phase would be formed in the composite, which will decrease the corrosion properties of the composites. The powder metallurgy technique could avoid the formation of the Al₄C₃ phase. In this work, three seized graphite flakes (150, 300, 500 μm) were used to investigate the effect of the graphite flake size on the strength and thermal conductivity of G_f/Al alloy composites. The 50%G_f /Al alloy (volume fraction) composites were fabricated by the powder metallurgy technique. The density of all the three G_f /Al alloy composites were similar to the theoretical density. The graphite flakes had a well bonding with Al alloy matrix without cracks and pores. The (001)_Gf basal plane of the graphite flakes were almost parallel to the circular plane (xy plane) of the composites ingot. However, for the small graphite flakes, their (001)_Gf basal plane was not well parallel to the xy plane of the composite ingot due to the powder metallurgy process. For the large graphite flakes, they exhibited a good orientation in the xy plane of the composite ingot. The strength of the G_f /Al alloy composites decreased with the increase of the graphite flake size. For the 150 μm graphite flake, the bending strength of the G_f /Al alloy composite was 82 MPa. However, for the 500 μm graphite flake, the bending strength of the composite decreased to 39 MPa. Due to the low strength between the layers of the graphite flake, the cracks were prone to expand in the graphite flake. As the size of the graphite flake increased, this phenomenon became more obviously. It is easy to observe that the graphite flakes peeled off on the fracture surfaces. When the size of the graphite flake increased from 150 μm to 500 μm, the thermal conductivity increased by 63%. The highest thermal conductivity was 604 W/(mK). The interfacial thermal conductance (h_c) of the composites were calculated by the Maxwell-Garnett type effective medium approximation model. The h_c of 300 and 500 μm graphite flake G_f /Al alloy composites were slightly lower than the theoretical value (calculated by the acoustic mismatch model). However, the h_c of the 150 μm graphite flake G_f /Al alloy composite was lower than that of the theoretical value. Besides the size of the graphite flakes, the shape, distribution and defect of the graphite flakes also influenced the thermal conductivity of the composites.