The martensites in steel T8A containing 0.8 pct C were examined by light microscope and TEM after martensitic austemper. There were boundary plate martensite, 12251 , butterfly martensite, { 2251, plate martensite, { 1 1 1 ], plate martensite, { 111 ], butterfly martensite and ( Ill], lath martensite observed in the steel. With the decrease of transformation temperature, the morphologies of the martensites formed changed from butterfly or plate to lath, the substructures from partial twin to dislocation and the habit planes from 12251, to ill1 1 ,. All these changes were controlled by the relief of elastic strain energy in the parent phase austenite. In a microregion of lath martensite, the martensites nucleate on two independent { 1111, planes respectively, and were formed in the way of filling in. The morphology of 1 1111, plate martensite found in this paper was similiar to that of thin plate martensite, bu it was mainly parallel and its substructure was partial twins.