A polyester resin with a superior heat-resistance and color as well as high content of a compound derived from biomass due to copolymerization of lactic acid or a compound derived therefrom and isosorbide, and a method for preparing the same are disclosed. The polyester resin is copolymerized with diacid components including terephthalic acid; diol components including 1 to 60 mol % of isosorbide and 1 to 90 mol % of ethylene glycol with respect to total diol components; and 1 to 50 weight % of lactic acid or a compound derived therefrom with respect to total reactants for resin polymerization, wherein, the polyester resin has the repeated structure of diacid moiety derived from the diacid components, diol moiety derived from the diol components and hydroxy monoacid moiety derived from the lactic acid or the compound derived therefrom.