Based on the cubic model developed by Ashby, the effects of the topology structure of light metallic foam such as porosity and cell size on its effective thermal conductivities were investigated. Using the least thermal resistance method, the expression of effective thermal conductivities was derived, and the approximate analytic solution was first obtained by employing the simplified integral method. The distribution of heat flux across the open-cell metallic foam and the steady Fourier law were simulated by ABAQUS software in order to determine the influence of porosity and cell size on the thermal conductivities of the open-cell metallic foam materials. The finding of this paper indicates that the effective thermal conductivities of porous metallic foam almost linearly decrease with the increase of porosity, and they have no change with the cell size when the porosity keeps constant. The theoretical study agrees well with the numerical data. The research results are significant to the practical engineering application of metallic foams.