An anti-theft fastener with locking feature includes a nut, a nut casing, a lock casing and a lock. An engaging ball of the lock casing gets embedded in an engaging groove of the nut casing. The lock and its' casing are positioned inside the nut casing. An abutting portion of the lock no longer abuts against a positioning ball of the nut. The positioning ball is no longer embedded inside a positioning engaging passage of the nut casing, thereby preventing the nut casing from driving the nut except idling. Under locked condition, the fastener is unable to be mounted. To dismount the fastener requires turning the lock with a key until the engaging ball falls into a curved groove of the lock and exits the engaging groove. The positioning ball is pushed outward and embedded inside the positioning engaging passage, thereby unfastening the screw element.