A thermal transfer sheet having a back coating layer excellent in heat-resistance is provided by using a styrene-acrylonitrile copolymer as a resin constituting the back coating layer. A thermal transfer material having an excellent heat-resistance and having a back coating layer having considerable strength is provided by incorporating at least two species of heat-resistant particles having different particle sizes in the back coating layer, since the larger species of particle imparts excellent heat-resistance to the back coating layer, the smaller species of particle enhances the total amount of the fillers, and the larger and smaller species of particles function so as to provide a synergistic effect. A thermal transfer sheet having a back coating layer which is excellent in storability is provided by incorporation an alkylphosphate multi-valent metal salt into the back coating layer, since the thus prepared back coating layer has an excellent heat-resistance, does not contaminate another menber or thermal head without wearing the thermal head, and has an excellent slip property and an excellent dye barrier property.