NOVELTY - A polyester composition comprises metallic element chosen from manganese and calcium and elemental phosphorus, and has carbon content of 0.1-0.5 wt.%, sum content of metallic elements of 0.59-11.15, intrinsic viscosity of 0.65 or more, difference ( delta ) of crystallization temperature of polyester composition and glass transition temperature of polyester composition of 45-70 degrees C and tracking voltage of 330 V or more. The metal content (M) and elemental phosphorus content (P) in polyester composition satisfies relation (I).    USE - Polyester composition is used for biaxially-oriented polyester film (claimed) for solar cell back sheet. Uses include but are not limited to motor for water heater, car and air-conditioner, drive motors, magnetic-recording material, capacitor, packaging material, building material, photograph, graphical use and thermal transfer for hybrid vehicle.    ADVANTAGE - The polyester composition has excellent heat-and-moisture resistance, mechanical characteristics, tracking resistance, chemical resistance, moldability, concealability and electric-insulation property. The biaxially-oriented polyester film provided using polyester composition has excellent durability.    DETAILED DESCRIPTION - A polyester composition comprises metallic element chosen from manganese and calcium and elemental phosphorus, and has carbon content of 0.1-0.5 wt.%, sum content of metallic elements of 0.59-11.15, intrinsic viscosity of 0.65 or more, difference ( delta ) of crystallization temperature of polyester composition and glass transition temperature of polyester composition of 45-70 degrees C and tracking voltage of 330 V or more. The metal content (M) and elemental phosphorus content (P) in polyester composition satisfies relation of formula: 0.8 less than or equal to M/P less than or equal to 2 (I). M is calculated using relation of formula: M=0.5x M1+M2, where M1 is sum total content of calcium element and manganese element and M2 is content of alkali content. An INDEPENDENT CLAIM is included for manufacture of polyester composition, which involves adding acetic acid to carboxylic component and alcohol component, reacting, obtaining ester composition, adding phosphoric acid or phosphoric acid alkali metal salt, polymerizing and adding carbon black.