A method for fabricating microstrip line structure is disclosed. First, a substrate is provided, ground patterns are formed on the substrate, an interlayer dielectric (ILD) layer is formed on the ground patterns, contact plugs are formed in the ILD layer, a ground plate is formed on the ILD layer, and a signal line is formed on the ground plate. Preferably, the ground plate includes openings that are completely shielded by the ground patterns.