The first and second laws of thermodynamics can describe the heat build-up phenomena of rubbery materials during deformation. When the force is applied on rubber specimen, the rubber starts to deform and the work is stored as heat inside rubber. The temperature increase due to deformation can be scaled by the heat capacity as a function of the work. This can be well explained by the entropic process. It can be noted that the entropic process is the reversible heat process, but the most of rubber applications are based on the behaviours such as viscoelastic process, which are not reversible heat process. In this paper we measured the phenomenological behaviours of temperature changes on stretching and un-stretching rubber. As a result it was reduced to two processes, which are the reversible process and irreversible process. The non-equilibrium thermodynamics equation was newly derived in order to describe these processes. The reversible process can be associated with the configurational entropic process of rubber elasticity, but in the irreversible process it was suggested that the work is leaked as heat to the external system. Some of work is transferred into the fluctuation of the molecular motion inside rubber. The fluctuation is never recovered as the mechanical force and is mechanically observed as the stress relaxation. This gives us the new interpretation why rubbery materials can produce heat during deformation. (C) 2015 Elsevier Ltd. All rights reserved.