Morphological features of transgranular fracture facets in low temperature brittle fracture of 18Cr-18Mn-0.7N high nitrogen austenitic steel have been studied by means of scanning electron microscopy and their formation mechanisms have been discussed. The transgranular fracture facets are fairly coarse compared with intergranular fracture facets and annealing twin boundary fracture facets. There are parallel steps and river patterns on the facets. Dual-surface observation indicated that these patterns are parallel to {111} planes and of strict crystallographic features. Microstructure observation shows that there are a lot of planar deformation structures formed prior to low temperature transgranular fracture. Transgranular fracture originates from microcracks formed at the intersections of the deformation structures on different {111} planes. These microcracks propagate toward adjacent microcracks on different {111} planes, forming transgranular fracture facets with steps and river patterns.