An interface card holder mounting arrangement includes an interface card holder attached to the vertical rear side wall of a casing for a computer mainframe, the interface card holder having hooks raised from a flange thereof extended out of an opening on the casing, a clamping plate having a tail end inserted into an insertion slot on a partition wall inside the casing and an angled clamping head clamped on the flange of the interface card holder, and a retainer plate which secures the interface card holder and the clamping plate to the vertical rear side wall of the casing, the retainer plate having one end hooked on the hooks of the interface card holder, a flange stopped at the angled clamping head against the flange of the interface card holder, and a flat tail fastened to a screw hole at the vertical rear side wall of the casing by a screw.