Principle of thermal printer is the pale material (usually paper) covered with a layer of transparent film becomes dark (usually black or blue) film is heated after some time. Image is formed by heating, chemical reaction generated in the film. This chemical reaction is at a certain temperature. High temperatures can accelerate the chemical reaction. When the temperature is below 60 ℃, the film need to go through a long, even up to several years to become dark; and when the temperature is 200 ℃, this reflection will be completed within a few microseconds.

Thermal printer is selectively heated at a predetermined position on the thermal paper, which gave rise to the corresponding graphic. Heating is provided by a small electric heater with a heat sensitive material in contact with the print head. Heater arranged in the form of square dots or strips by the printer logic control, when driven, it creates a heating element with corresponding graphics on thermal paper. Controlled by the same logic circuit heating elements, but also controls the feed, which can print graphics on the label or the entire paper.

The most common thermal printers using fixed dot matrix print head for a heating tape, the print head has 320 square points, each point is 0.25mm × 0.25mm. With this dot matrix printer can print at any point in the position on the thermal paper. This technique has been used on paper printers and label printers.