Abstract: The 3-piece metal door frame comprises two vertical side metal door frame members and a horizontal top metal door frame member. An improved joining system comprises a pair of L-shaped joining brackets provided on one of the door frame members, the joining brackets being adapted to receive matching screw fasteners provided in connection with an adjoining door frame member. When the screws are received in the brackets they allow the adjoining frame members to achieve a secure and close mitre fit. Guiding tabs are provided at the top of the jam of each vertical side door frame member and are adapted to be received in matching slots provided in each end of the jam of the horizontal top metal door frame member. When two of the door frame members are joined to form a joint the guiding tabs help to keep the door frame members correctly aligned.