||Foaming Processes of a rigid polyurethane foam (RPUF) with open-cell structures was investigated using a lithium salt of 12-hydroxystearic acid (Li-12HSA) as the reactive cell-opening agent. Temperature changes with time during the foaming was measured in the center of open-mold. As the results, An inflection point was observed at about 40 sec after start of foaming process, indicating the formation of effective cell structures which is starting time for the thermal insulation during the foaming process. Peak temperature of samples with Li-12HSA showed ca. 160 ℃, which was 10 ℃ higher than that samples without Li-12HSA. In addition, time to reach the peak temperatures was also observed as faster, showing that Li-12HSA could play a role for a catalyst as well as a novel reactive cell opening agent.