This paper describes a relationship between a water content of printing paper and an equivalent thermal conductivity that affects to printing quality of printers using the heat such as laser printers and direct thermal printers. Our study targets to describe the relationship between the water content and the thermal conduction in the printing process. Especially in this report, we firstly investigated a relationship between the humidity around the paper and the change of the water content of the thermal paper. The dried thermal paper was mounted in the constant temperature and humidity chamber and the time history of the change of the paper weight was measured by using the precision balance. It is found that the water content of the thermal paper is dependent on the humidity. However, even if the humidity around the paper is changed, the water content of the paper is rapidly saturated according to the humidity. Therefore, from the viewpoint of the thermal design of the printing process, the transient change of the water content of the paper may not become important against the value of the humidity itself. In addition, the relationship between the water content of the paper and the thermal conductivity was investigated through a combination of the 1-dimensional thermal conduction experiment in the constant temperature and humidity chamber with the thermal network analysis.