In a thermo-shrinkable polyester film, the thermal shrinkage ratio in a main shrinking direction is about 20% or more after being treated in water of about 70.degree. C. for about seconds, about 35 to about 55% after treated in water of about 75.degree. C. for about 5 seconds, and about 50 to about 60% after being treated in water of about 80.degree. C. for about 5 seconds. After the film is shrunk by about 5% by treatment in water of about 75.degree. C. for about 10 seconds, the probability of the elongation at rupture of the film in the direction perpendicular to the main shrinking direction being about 20% or less, is about 10% or less. The ratio of .tau..sub.1 /.tau..sub.2 is about 0.05 to about 0.45, where .tau..sub.2 is the stress of shrinkage in a main shrinking direction of the film at about 90.degree. C., and .tau..sub.1 is the stress of shrinkage in a direction perpendicular to the main shrinking direction of the film at about 90.degree. C. The sealing strength of the film is about 1.2 kg/15 mm of width or more.