The volume expansion behavior of low-density polypropylene foams in extrusion is investigated in this paper. Since escape of blowing agent from the foam would cause the foam to contract, and to have low expansion, efforts were made to prevent gas loss during foaming. The basic strategies to the promotion of a large volume expansion ratio are:​ to use a branched material for preventing cell coalescence;​ to use a long-chain blowing agent with low diffusivity;​ to lower the melt temperature for decreasing gas loss during expansion;​ and to optimize the processing conditions in the die for avoiding too-rapid crystallization. Use of a branched polypropylene resin was required to achieve large volume expansion because prevention of cell coalescence will retard gas loss from the extruded foam to the environment. The foam morphologies of linear and branched polypropylene materials at various processing temperatures were studied using a single-screw tandem foam extrusion system and their volume expansion behaviors were compared. Ultra lowdensity, fine-celled polypropylene foams with very high expansion ratio up to 90 fold were successfully produced from the branched polypropylene resins.