An important indicator of sealing fastness for composite plastic bags is heat sealing strength. Its failure mode has not been explained in testing standards. According to QB/T 2358—1998 of Test Method for Heat Sealing Strength of Plastic Film Packaging Bag, experiments for heat sealing strength were conducted with plastic packaging bags of different materials，such as PET/Al/PA/RCPP film, OPP/VMPET/PE film, PET/RCPP film, PET/Al/PA/RCPP film and aluminum laminated film. Five failure modes were concluded as: heat seal peeling, heat seal edge fracture, film fracture, film layered, separation and tearing. Analysis on the causes of failure combined with the measurement graphs and failure mode patterns could effectively guide and improve the production process of plastic bags.