The effect of thermomechanical treatment on thermal expantion and melting point of Fe-30%Ni-0.35%C alloy was investigated. The dimention changes of the ausformed martensite and the marformed martensite were decreased with increasing deformation degree in the range of 25∼350℃ prior to reverse transformation but became larger in the range of 500∼800℃ after the reverse transformation. The dimension change and the thermal expansion coefficient were reduced in the order of the deformed austenite, the marformed martensite and the ausformed martensite in the range of 25∼800℃. Therefore, the ausforming treatment is more effective than the marforming treatment in improving the heat-resistance. The melting points of the deformed austenite, the ausformed martensite and the marformed martensite were lowered as either the heating rate or the degree of deformation was increased.