It is an object of the present invention to provide a thermal transfer sheet which has high adhesion strength of a dye layer to a base and a high reflection density, and has a good releasing property from a image-receiving sheet in printing even after being stored in the conditions of high temperatures and high humidity, and can attain adequately satisfactory printed substances having high sharpness of thermal transfer images.The present invention pertains to a thermal transfer sheet, wherein an under coat layer containing a thermoplastic resin and colloidal inorganic pigment ultrafine particles and a dye layer are formed in order on a face on one side of a base and a thermal transfer sheet, wherein an under coat layer formed by using colloidal inorganic pigment ultrafine particles and silicate or aluminum alcoholate and a dye layer are formed in order on a face on one side of a base.