Graphite and aluminum powders are typical aerosol particles whose infrared (IR) extinction properties are important to the IR obscuring performances of the smoke ammunitions containing these powders. Mass extinction coefficients (MECs) of the two materials with different sizes are measured in a smoke chamber. The relationship between the powder size and the IR extinction property is analyzed, with the difference between their extinction properties being discussed. Moreover, test accuracy is studied at different temperatures of the target and the background. The results show that the IR extinction property gets better as the size of the graphite powder decreases. Due to its better dispersivity, larger radius-thickness ratio and larger IR refractive index, aluminum powder shows better extinction properties than graphite powder of the same size. The IR MECs of aluminum powder (1 000 meshes) are respectively 1.78 m²/g (3.7- 4.8 μm) and 2.01 m²/g(7.5-14 μm), with those of graphite powder of the same size being 1.02 m²/g and 1.01 m²/g respectively. In addition, test results are found to be closely related to the initial temperatures of the target and the background, showing the necessity of appropriate settings of the temperature.