A method for forming a seal ring is disclosed. First, a substrate including a MEMS region, a logic region and a seal ring region is provided. Second, a trench is formed in the MEMS region and multiple recesses are formed in the seal ring region. An oxide fills the trench and the recesses. Later, a MOS is form in the logic region and a dielectric layer is formed on the substrate. Then, an etching procedure is carried out to partially remove the dielectric layer and simultaneously remove the oxide in the multiple recesses completely to form a seal ring space. Afterwards, a metal fills the seal ring space to from the seal ring.