Polymeric foams are one of the solutions to improve the properties of polymers. Researchers have shown that by increasing cell density and decreasing foam density and cell size, better properties can be achieved. Process conditions are effective in changing the properties, one of which is the foaming agent. Therefore, in the present research, while maintaining the material and process conditions, high-density polyethylene granule grade 54404 was used as the background phase material. The production of the samples is done in two stages; with the carbon dioxide foaming agent and nitrogen foaming agent. The effect of the type of physical foaming agent on the structure of extruded polymeric foams is investigated. Results showed that in the production of the sample with the physical foaming agent of carbon dioxide gas, the foam density was 0.420 g/cm³ and the expansion ratio was 2.17 times and the cell density was 2.78×10⁴ cells/cm³. Furthermore, the results of physical foaming agent of nitrogen gas showed that the foam density was 0.628 g/cm³ the expansion ratio was 1.45 times, as well as the cell density was 4.32×10⁴ cells/cm³. In the sample production with carbon dioxide gas, the higher solubility property of carbon dioxide gas compared to nitrogen in the polymer matrix caused the production of foam with low foam density and higher expansion ratio. Also, in the sample production with nitrogen gas, due to the high nucleation property of nitrogen compared to carbon dioxide, it led to the production of samples with increased cell density.