The present disclosure provides conductive plug structures and fabrication methods thereof. An exemplary fabrication process of the conductive plug structure includes providing a substrate; forming a mask layer having an opening on a surface of the substrate; etching the substrate to form a contact hole using the mask layer as an etching mask; etching the mask layer to increase a feature size of the opening; forming an insulation layer on an inner surface of the opening, an inner surface of the enlarged opening and a surface of the mask layer to have more edge corners, a thickness of the insulation layer being greater than a thickness of the remaining mask layer; forming a conductive layer filling the contact hole on the insulation layer; and planarizing the conductive layer and the insulation layer until a surface of the mask layer is exposed.