Exemplary metal line structure and manufacturing method for a trench are provided. In particular, the metal line structure includes a substrate, a target layer, a trench and a conductor line. The target layer is formed on the substrate. The trench is formed in the target layer and has a micro-trench formed at the bottom thereof. A depth of the micro-trench is not more than 50 angstroms. The conductor line is inlaid into the trench.