The effect of several processing additives and processing parameters on the foam extrusion of rigid poly(vinyl chloride) (PVC) was investigated. The metallic thermal stabilizer played a role as auxiliary nucleating agent and promoted decomposition of ADCA. Additional nucleating agent tad little effect on cell size and distribution. Two acidic nucleating agents, citric acid and boric acid, promoted decomposition of ADCA and SBC. They lowered foaming temperature and entranced foaming efficiency. The decomposition of ADCA was promoted almost in proportion to acidity of nucleating agent. An increase of temperature increased cell size but decreased cell frequency, but an increase of shear rate increased cell number along with cell size. A decrease of die L/D ratio enhanced foaming efficiency and increased cell size.