The theory of the reflow applied to metallization process was studied, and the factors affecting the reflow and the relation between the reflow and the grain growth were investigated. The driving force for the metal reflow is the difference in chemical potentials along the metal surface, and it causes the atom movement. On condition that metal interconnect is fabricated for semiconductor devices, surface diffusion is the primary atom movement mechanism. The metal reflow is influenced by reflow temperature, reflow time, reflow ambient, thin film thickness, thin film material, underlayer material, pattern size, and aspect ratio. It is supposed that the reflow characteristic varies according to the grain growth during the reflow, so the effect of the grain growth on the reflow should be considered.