  Thermal rectification which is a diode-like behavior of heat flux has been
studied over a long time. However, a universal and systematic physical
description is still lacking. In this letter, a perturbation theory of thermal
rectification is developed, which provides an analytical formula of the thermal
rectification ratio. It reveals the linear relationship between the thermal
rectification ratio and temperature difference. Furthermore, the
size-dependence of the thermal rectification relies on the specific form of the
thermal conductivity. In addition, several experimental and numerical
observations in previous literatures are well explained. This theory can be
applicable to any system in which a differentiable effective thermal
conductivity can be derived, and is helpful to unveil general principle for
thermal rectification.
