The foaming behaviour and cell structure development of linear and branched polyamide 6 (PA6) in a static foaming system were investigated using carbon dioxide (CO2) and nitrogen (N2) as physical blowing agents. Differential scanning calorimetry (DSC) analyses indicated on faster crystallization of branched PA6. The microstructure analyses showed that the foaming window of the branched PA6 was wider than that of the linear PA6. The cell density was also higher in the case of branched PA6 as compared to the linear one which can be attributed to its higher crystallization temperature. We surmise that the formation of crystals in branched PA6 played a crucial role in the enhancement of its cell density. Furthermore, the expansion ratio of the branched PA6 foams was significantly larger than that of the linear PA6 using CO2. This can be attributed to the higher melt strength of the branched PA6. The comparison of rheological behaviours of the linear and the branched PA6 showed a higher complex viscosity of branched PA6 which might have played a role in achieving high expansion. Unlike the branched PA6 foams, severe cell coalescence was observed in the linear PA6 foams leading to deteriorated cell structure and low expansion due to gas loss. On the contrary, we successfully achieved high cell density branched PA6 foam with expansion ratio of about 20 times with highly uniform cell structure. In general, higher expansion ratio was achieved for PA6 foamed with CO2 as blowing agent compared to N2.