Low Density Polyethylene (LDPE) is blended with High Density Polyethylene (HDPE) and three types of Polypropylene (PP) having different melting index (MI), respectively. The compatibility of LDPE/HDPE blends is characterized by using thermal analysis and rheology methods. Differential scanning calorimeter (DSC) traces and rheology methods confirmed their good compatibility. For LDPE/PP blends, the incompatibility has been widely acknowledged in the literature. The distribution of the PP phase in the blend is investigated using polarized optical microscopy (POM). It is found that the phase structure is closely related to the blend composition and the viscosity ratio of the blend components. Extrusion foaming of the blends is conducted using a single extruder fitted with a CO2 gas injection system. Under similar foaming conditions, the compatible LDPE/HDPE blends all generated a uniform cell morphology and achieved roughly the same expansion ratio. Although the incompatible interface is beneficial to cell nucleation, the LDPE/PP blends did not achieve satisfactory foaming performance. To determine the differences in the foaming behavior of the two blends, the viscoelastic properties and diffusion coefficients of supercritical CO2 in the blends, were accurately measured via rheology and magnetic suspension balance (MSB) methods. The results indicate that the viscoelastic properties did not show a dominant role in determining the foaming behavior, by contrast, CO2 diffusion is found to be the key factor affecting foaming performance especially in the case of a co-continuous phase structure. (C) 2016 Elsevier B.V. All rights reserved.