Abstract: An assembly jig for a prefabricated door unit comprises two elongated beam members which may be secured to a support surface in a parallel spaced apart orientation. A pair of holding members are detachably coupled to each of the beam members in a spaced orientation substantially equal to the width of the door unit. Each of the holding members has a recess for receiving a first pair of components of the door frame, thereby allowing the other door frame components to be nailed thereto.