Abstract: Extrusion foaming parameters (i.e. nucleating agent, gas injection and die pressure) of PP with butane gas were investigated by an orthogonal experimental method. Cell structures obtained under different process conditions were determined by scanning electron microscopy. The effects of process parameters on the expansion ratio, average cell diameter and cell density of PP foams were analyzed, and the relevant mechanisms were discussed. The foaming process was further optimized with the Design Expert software, and the resulting process parameters were confirmed by the relevant experiments. The results indicated that the PP foams achieved a high expansion ratio, a high cell density and a large average cell diameter when the content of nucleating agent, die pressure and gas injection amount were set to 1.4 wt %, 4.66 MPa and 7 %, respectively. The experimental results also verified the theoretical predication with a deviation lower than 5 %.