An experimental study was performed to obtain some design data for a cool storage system using HCFC-141 b clathrate. Experiments on investigating the possibility of using the clathrate storage system as an air condition for a building were performed. The clathrate can store energy of night excess power at a cool storage density and extract it as cool heat when the energy becomes necessary in the daytime. Commercial cool storage system using CFC (Chloro Fluoro carbon)-11 (CCl3F) clathrate have already been developed. But now, the usage of CFC-11 is strongly restricted from the stand point of global environmental protection, hence an alternative medium for CFC-11 is necessary. In this study, HCFC -141 b was selected as an alternative medium. The formation tests for HCFC-141 b clathrate were carried out in order to obtain some design data for cool storage systems. The following conclusions were obtained: (1) a cool storage system using HCFC-141 b clathrate is a more effective heat storage system than ice; (2) addition of 0.05 wt% surfactant allowed an ideal clathrate formation/decomposition up to 40 wt% of clathrate concentration; and (3) addition of ethyleneglycol allowed the clathrate formation/decomposition temperature to decrease, resulting in the acquisition of more cooled water.