Abstract: Crack initiation and propagation and fracture surfaces in low temperature brittle fracture of an 18Cr-18Mn-0.7N high nitrogen bearing austenitic steel were studied by means of scanning electronic microscopy. In the fracture,micro crack initiated at grain boundary and annealing twin boundary. Then the cracks propagated by coalesce through grain. As a result,three kinds of fracture facet exhibited on the fracture surface,i.e.,smooth and flat facet,smooth curved facet and very rough facet,induced by annealing twin boundary cracking,intergranular cracking and transgranular cracking,respectively.