Two chemical blowing agents with different decomposition mode, azodicarbonamide (ADCA) and sodium bicarbonate (SBC), were selected, and foaming process of rigid poly (vinyl chloride) (PVC) was investigated. At the same usage level, ADCA gave higher degrees of nucleation and growth of microbubble than SBC. Further, the decomposition mode and gas properties had a profound influence on foaming process and foam properties. ADCA exhibited high initial bubble expanding rate under ambient pressure and yielded almost round to elliptical cells with even size distribution, while SBC required some induction time for bubble expansion and yielded irregularly shaped cells with wide size distribution. Of the two chemical blowing agents, ADCA yielded foams with better mechanical properties than SBC.