Abstract: By using a slot rhcological die with a slit channel and I,abvicw data collection system, rhcological behavior of polypropylene/supercritical carbon dioxide mixture was studied at different temperatures and gas concentration. Critical foaming pressure of PP/COz system was also determined by utilizing a novel ultrasonic detecting system. It showed that the viscosity curves under different conditions have similar slope, and shift linearly down with increasing melt temperature and gas volume. When melt temperature was below 175℃，the critical foaming pressure increased with decreasing melt temperaturc.

Key words: in-line rhcology measurement, slit die, polypropylene, carbon dioxide, critical foaming prcssurc