Thin and continuous CVD Cu seed layer was successfully deposited on Ru under-layer by Cu oxide deposition and reduction method at 100°C with novel chemistry. Cu oxide was formed with Cu(hface)TMVS and H2O2 at 100°C, and this film was reduced with formic acid at 100°C. Deposited Cu oxide films were Cu2O that was confirmed by XRD and XPS. The morphology of oxide films showed smooth and continuous on Ru and Ta substrate. The reduced Cu film on Ru maintained good surface morphology, and no impurity was detected not only in the Cu film but also the interface between Cu and Ru. However, that on Ta had poor morphology by agglomeration of Cu film during reduction due to poor Cu wettability on oxidized Ta that was oxidized during oxide deposition. The readiness of reduction is very important merit of using Ru under-layer for this oxide deposition and reduction process. The oxide deposition and reduction method on Ru under-layer can be a promising candidate for thin and continuous seed layer deposition method.