Summary:
Features include 1) a fan-shaped or kidney-shaped, thin, leathery fruitbody narrowed into a stemless base and growing on old logs, 2) an upper surface at first orangish, tomentose-downy, faintly zoned, when mature dull dark red, bald, and densely radiate-wrinkled, 3) a spore-bearing surface underneath that is yellow, becoming brick color when old, 4) microscopic characters obscure in that only 2 spores seen, but no cystidia, composed of an intermediate layer of longitudinal, densely arranged, thick-walled, rigid hyphae 3-3.5 microns wide, bordered on the upper side by an opaque brown layer that gives the color to the cap, and on the lower side hyphae curving into a hymenial layer.

Microscopic:
spores up to 7 x 2-2.5 microns present, but "may not belong for only 2 seen"; no cystidia, gloeocystidia, or conspicuous conducting organs seen; in structure about 800 microns thick, composed of 1) an intermediate layer of longitudinal, densely arranged, thick-walled, rigid hyphae 3-3.5 microns wide, 2) bordered on the upper side by an opaque brown layer 60 microns thick that gives the color to the cap, and 3) "curving on the lower side into a hymenial layer 300 microns thick", (Burt)

Notes:
Stereum atrorubrum is known only from the holotype specimen, found in BC, (Ginns).