A thermal paper with a fluorescent security mark printed on the thermosensitive coating that is responsive to wavelengths in the range of 200 nanometers to 400 nanometers. The mark is either printed on the thermosensitive coating directly or on an optional protective top coat positioned over the thermosensitive coating. Methods of preparing the thermal paper comprise printing a solution, dispersion or emulsion of a fluorescent compound on the thermal paper by flexographic printing.