A method for fabricating MEMS device includes: providing a single crystal substrate, having first surface and second surface and having a MEMS region and an IC region; forming SCS mass blocks on the first surface in the MEMS region; forming a structural dielectric layer over the first surface of the substrate, wherein a dielectric member of the structural dielectric layer is filled in spaces surrounding the SCS mass blocks in the MEMS region, the IC region has a circuit structure with an interconnection structure formed in the structural dielectric layer; patterning the single crystal substrate by an etching process on the second surface to expose a portion of the dielectric member filled in the spaces surrounding the SCS mass blocks; performing isotropic etching process at least on the dielectric portion filled in the spaces surrounding the SCS mass blocks. The SCS mass blocks are exposed to release a MEMS structure.