Abstract: The present disclosure provides a fastening system for using in a flexible tonneau cover system. In one aspect, the fastening system comprises a corner member connected to a end rail, and a connecting member connected to an side rail. The corner member comprises a positioning tab that is received in a positioning groove formed on the connecting member. In another aspect, the fastening system comprise a corner member with a recess portion. When the corner member is connected with a side rail at an end thereof, the recess portion and the end of side rail form an accommodating groove. In another aspect, the fastening system comprise a fastening element coupled to an elongated slat. A reinforce tab is coupled with the fastening element, and wraps over an end of the elongated slat to extend at a bottom surface of the elongated slat.