The effect of austenitizing time on the formation of martensite in Fe-24.5%Ni-4.5%Si alloy has been studied by means of transmission electronmicroscope (TEM), scanning electronmicroscope (SEM) and Mossbauer spectroscopy technique. TEM and SEM observations revealed that the martensite morphology was found to be closely dependent on the austenitizing time. The orientation relationship between austenite and thermally induced martensite was found as the Kurdjumov-Sachs type. The volume fraction changes of martensite and austenite phases, the hyperfine magnetic field of martensite phase and isomery shift values have been determined by Mossbauer spectroscopy. The Mossbauer study also revealed that the martensite volume fractions increased with increasing austenite grain size.