Heat sealing is an important technique of bonding thermoplastic materials. However, only heat seal strength has been used to evaluate the bonding of thermoplastic material for a long time. The customers of the packaging products hope for both easiness of opening and completed seal. Heat sealing technology is the most economical for easy opening the package. Manufacturers could not serve customers requirements although they apply the conventional standards such as ASTM F88, F2029, JIS Z 0238. Indeed, the heating temperature at the interface of bonding affect mainly to heat seal strength. The objectives of this research are proposed testing method and analysis of the Openability and Closability of heat seal strength. The methodology used in this research is divided into 2 testing methods; the testing method of heat seal strength with accurate heating temperature, and the testing method of elements generating heat sealing. The first testing method is used for identify the accurate heat seal strength by regarding peculiar thermobonding performance. The second testing method is used to identify the close functions and comprehend the peculiar characteristics of materials. With this proposed methodology, it is possible to determine the closing function of heat sealing based on reliable characteristics of material. The analysis of the peeling energy of adhesive bonding of heat sealing will be discussed for Closability in this research. Functional Heat Seal Strength (FHSS) is introduced to determine the generation of pinhole and polyballs by observation the failure of heat seal testing. Experiments in this research are conducted by using propose innovative testing method with MTMS (The Measurement of temperature at melting surface). PE/70 material widely used in local market is used in the experiment. Also, this research challenge evaluation method for Closability of heat sealing of thermoplastic materials. The result shows that Peel seal can guarantee closing ability of material for packaging.