Flame retardant polyester compositions suitable for calendering may be prepared from polyesters having crystallization half time of at least 5 minutes, a plasticizer, a phosphorus-containing flame retardant miscible with the plasticized polyester, and an additive effective to prevent sticking of the polyester to the calendar rolls. Also disclosed are processes for flame retardant films or sheets by calendering the above compositions and the flame retardant films or sheets produced therefrom. These films and sheets have excellent appearance, flexibility, and flame retardancy, and can be used in a wide range of decorative and packaging applications.