Radiation is one of the three heat transfer modes. Different from conduction and convection, radiation does not require substances as media between the heat source and the heated object. At the molecular level, radiation transforms heat energy by electromagnetic waves. As long as the electromagnetic wave is not blocked, radiation will occur between the objects. To quantify the level of transformation of electromagnetic wave, a parameter called “view factor” is used in heat transfer simulation. In this course, view factor will be discussed in detail for different conditions of surfaces for radiation. For conduction-based solver is the focus, radiation is considered as a nonlinear boundary condition controlled by several parameters. Both the formulation of the radiation and the important parameters controlling radiation will be explained in this course.