The disclosure provides a method for manufacturing a memory device, including: providing a plurality of gate structures formed on a substrate, wherein the gate structures comprise a cap layer disposed on the top of the gate structure, and each two adjacent gate structures are separated by a gap; blanketly forming a polysilicon layer on the substrate to fill the gap; performing a planarization process to the polysilicon layer, obtaining a polysilicon plug; and performing an oxidation process after the planarization process, converting a part of the polysilicon plug and a residual polysilicon layer over the gate structure to silicon oxide.