Flexible packaging by laminated film has been widely utilized for the distribution of foods, medical supplies and various every-day goods in recent years. One drawback of this packaging is high risk of fracture in the process of distribution due to temperature changes and/or externally applied forces. The present study discussed the tensile strength of heat-sealed part where most failure events have been reported.In order to investigate the effect of seal layer thickness and seal material on the tensile strength of the sealed part, both the test temperature (T) and the loading tate (dP/dt) were varied for 263 K (-10°C) < T < 323 K (50°C) and dP/dt = 3.33 10 - 3 , 0.77 and 4.08 m/s. The tensile strength becomes insensitive to the seal layer thickness when increasing the load rate. While, it is improved by a change in material of the seal layer. Peeling behavior of the seal was also observed by a coloring agent.