In this paper, self-heating effect in newly introduced Vertical channel Field Effect Transistors (VFETs) is investigated and discussed, and several architecture parameters such as channel width, number of channels affecting thermal reliability of VFET are studied through simulations. In addition, a study on the change of thermal properties is conducted when air gap/spacer is used to reduce the influence of the parasitic capacitance. It is illustrated that VFET shows high lattice temperature and thermal resistance increase from changes in such architecture parameters, air gap in BEOL (Back End of Line), and air spacers. Additionally, the lattice temperature imbalance between channels can be mitigated by adjusting the inter-channel spacing of a multi-channel VFET.