Carbon deposition is an undesired side product, significantly inhibiting chemical reactions as well as leading to performance degradation. To efficiently prevent carbon deposition, the underlying mechanism needs to be elucidated. In this work, we have employed Raman spectroscopy to characterize the formed carbon deposits under different conditions inside a high temperature furnace heating system with a built-in U-shape quartz tube. The formation of graphite/amorphous carbon deposition was related to the temperature-dependent diffusion rate of carbon atoms, as well as the surface state of stainless steel. These investigations would help reduce unnecessary carbon deposition not only in ethylene but also in other industrial productions such as fuel cells.