Abstract: The effect of austenite ageing on martensite transformation in an iron and nickel alloy was studied. Attention was paid to the variation of dislocation configurations in austenite before martensite transformation. The clusters or precipitates produced during austenite ageing and the alteration of dislocation configurations affect together the consequent martensite transformation.Dispersed fine clusters and precipitates cripple nucleus role of special dislocation configurations and drop M. Coarse precipitates make Ms rise again. Densely piled up dislocations in nuclei lead to rise of Ms. All the changes in austenite are of benefit to small martensite plates.