To reduce the number of synthesis and layout iterations, we present a new delay optimization technique, which inserts buffers based on back-annotated detailed routing information. During optimization, inserted buffers are assumed to be placed on the appropriate location of original wires so as to calculate accurate wire RC delay. With forward annotated location information of inserted buffers, the layout system attempts to preserve patterns of original wires using the ECO technique. Our experimental results show that this technique combined with the conventional gate sizing technique achieves up to 41.2% delay reduction after the initial layout.