Abstract: To study the influence of diameter and paving location on the cooling effect of ballast embankment, the one dimensional tests were carried out on the three materials of ballast, cobble and sandy gravel with single, compound and mixed structures in an insulated box of 50cm in length and width and 65 cm in height under the periodic fluctuation of air temperature at the top. Results show that for the single structure, the natural convection is appeared respectively in the ballast with the diameters of 2–4 cm, 4–6 cm, 6–8 cm and 10– 15 cm, among which the ballast with diameter of 4–6 cm provides the best cooling effect. For the compound and mixed structures, the cooling effects are weakened. The mean temperature of ballast is decreased with increase of its thickness. To use the mechanism of natural convection sufficiently we suggest that the ballast with the diameter of 4–6 cm could be used as filling material for the embankment in permafrost regions and the paving location should be at the top of the embankment. The compound and mixed structures of different materials, such as ballast, cobble and sandy gravel, should not be used in permafrost regions.