In terms of thermal properties of polymers, there are rubbers and gels, which are very soft even at low temperatures, while there are also epoxy resins that are hard even at high temperatures. In other words, this large degree of freedom on properties is one of the major reasons why polymeric materials have spread so widely around the world. Although plastics are generally regard as materials that are cheap, lightweight, and easy to process and use, their performance may not be as poor as one might imagine. Polymers, whether natural or man-made, have a wide range of high-performance characteristics at the stage of synthesis. Research is also underway to add new performance to these materials. In order to develop high-performance polymers by maximizing the expression of these performances, it is necessary to identify the ultimate limit of the performance and to understand the structure for the expression of the performance, even though it may seem to be a long way off. In fact, there are polymers that are stronger and show more heat conduction than iron. Here, we will describe on the mechanical and thermal properties of polymers from the viewpoint of their structure.