Thermal paper is a special coated paper that looks similar to ordinary white paper. The surface of the thermal paper is smooth, which is made of plain paper as a paper base, coated with a layer of thermal chromonic layer coated on the surface of the plain paper. The chromonic layer is made of adhesive, developer, leuco dye (or The composition of the leuco dye is not separated by microcapsules, and the chemical reaction is in a "latent" state. When the thermal paper encounters a hot print head, the color developer and the leuco dye at the place where the print head is printed are chemically reacted and discolored to form a graphic image.

When the thermal paper is placed in an environment above 70 ° C, the heat-sensitive coating begins to change color. The reason for its discoloration is also to talk about its ingredients. There are two main types of heat sensitive materials in thermal paper coatings: one is a leuco dye or a leuco dye; the other is a color developer. This type of thermal paper is also referred to as a two-component chemical type thermal recording paper.

Commonly used as leuco dyes are: crystal violet lactone (CVL), fluoran system, colorless benzoylmethylene blue (BLMB) or spiropyran system of triphenylmethyl phenyl hydrazine system. Commonly used as a color developer are: para-hydroxybenzoic acid and its esters (PHBB, PHB), salicylic acid, 2,4-dihydroxybenzoic acid or aromatic sulfone.

When the thermal paper is heated, the leuco dye reacts with the developer to produce a color. Therefore, when the thermal paper is used to receive the signal printing on the fax machine or directly printed by the thermal printer, the graphic is displayed. Since there are many kinds of varieties used as leuco dyes, the color of the written characters is different, and there are blue, magenta, black, and the like.