Stretched polyester film and method for preparing the same

A stretched polyester film, which comprises a single crystalline polyester resin in which a diol component and a dicarboxylic acid component are polymerized,

• wherein the single crystalline polyester resin consists of a single polyester material,
• wherein a heat shrinkage rate of the stretched polyester film at 80° C. in main shrinkage direction is 40% or more, and, upon a first thermal treatment and a second thermal treatment, a first melting temperature (Tm1) and a second melting temperature (Tm2) appear when the stretched polyester film is analyzed by differential scanning calorimetry (DSC),
• wherein the first melting temperature (Tm1) is 150 to 230° C., the second melting temperature (Tm2) is 150 to 230° C., and the first melting temperature (Tm1) and the second melting temperature (Tm2) are different from each other,
• wherein a difference (|Tm1−Tm2|) between the first melting temperature (Tm1) and the second melting temperature (Tm2) is in a range of 10 to 60° C.,
• wherein the first thermal treatment temperature is 60 to 105° C.,
• wherein the second thermal treatment temperature is 140 to 220° C., and
• wherein a ratio (H1/H2) of a first heat of fusion (H1) at the first melting temperature (Tm1) to a second heat of fusion (H2) at the second melting temperature (Tm2) is 0.1 to 10.