The heat-sealing strength of pouch film greatly affects the reliability of the lithium ion secondary battery. In this paper, the researchers investigated and evaluated the properties of the heat-sealing strength of pouch film, such as heat, pressure, time, thickness of the heat-seal, and the polypropylene material. The heat-sealing strength showed a high value at 180℃ for 3 seconds. However, under the conditions of higher temperatures and longer times, deformation and bulging of polypropylene were observed. The heat-sealing strength tended to increase when decreasing heat-seal thickness. The heat-sealing strength varied according to the type of polypropylene. In addition, to avoid defects that may have occurred in the process of manufacturing the lithium ion secondary battery, the heat-sealing strength in the state where the impurities remained was evaluated.