Thermal sensation is an important channel for getting information. Whereas there has been significant progress in the development of technologies controlling the heat to simulate thermal sensation, there is a lack of understanding about how to utilize it in designing interactive systems. We propose the term "thermal expression'' to signify the activity of controlling a particular amount of heat with intention or the heat generated as a result of that activity to crystallize our new view to the heat and a systematic way of thinking about the quality and the structure of the thermal expression. Focusing on the interpersonal communication domain, we explored the expressive quality of the heat through a prototyping approach deploying the heat-based communication devices that we developed in the daily life environment of normal users. We discovered the characteristics of thermal expression and the design opportunities for thermal application development, especially for interpersonal communication. The emotional value and the role of heat as a communication medium are discussed.