Abstract: A two-piece anti-loosening nut assembly includes a nut and a washer. The nut is configured to threadedly engaged with a bolt and has a main body including an annular outer flange extending outwardly and radially from an end periphery thereof. The annular outer flange has an annular flange surface formed with a cam structure. The washer is fixedly disposed on the nut, and has a lower ring surface facing the annular flange surface and formed with a cam structure engaged with the cam structure of the annular flange surface, and an upper ring surface opposite to the lower ring surface and formed with a serrated structure configured to bite into a surface of a workpiece.