Abstract: Influences of solid solution treatment and ageing processing on microstructure of 0Cr21Ni6Mn9N nitrogenous austenitic stainless steel were investigated by metallographic optical microscopy technigue. The results show that after solution treatment at 1 100 ℃, the grains of 0Cr21Ni6Mn9N steel cold drawn tube grow, more twins appear and the layer thickness of twin increases, and there is no precipitates on grain boundary. However, after ageing treatment at 650 ℃, the grain size has no obvious difference with that after solid solution, and there are a lot of black acicular M23C6 phases precipitating on grain boundaries. After ageing treatment at 850 ℃, the grains grow up again because big grains swallow small grains, the M23C6 phases on grain boundaries decrease compared with those at 650 ℃.