A method of forming a shallow trench isolation structure includes steps of providing a substrate having a patterned mask layer formed thereon, wherein a trench is located in the substrate and the patterned mask layer exposes the trench. Thereafter, a dielectric layer is formed over the substrate to fill the trench. Then, a main polishing process with a first polishing rate is performed to remove a portion of the dielectric layer. An assisted polishing process is performed to remove the dielectric layer and a portion of the mask layer. The assisted polishing process includes steps of providing a slurry in a first period of time and then providing a solvent and performing a polishing motion of a second polishing rate in a second period of time. The second polishing rate is slower than the first polishing rate. Further, the mask layer is removed.