This work was carried out in order to investigate the effect of grain size on martensitic transformation temperature and morphology of Fe-27%Ni-0.27%C alloy. The martensitic transformation temperature was raised with increasing the austenitizing temperature within the range from $750^{circ}C$ to $1200^{circ}C$, owing to the grain growth, vacancy concentration. It was observed that the larger was the austenite grain, the higher was the martensitic transformation temperature. The influence of the austenite grain size was similar to that of the austenitizing temperature. The morphology of martensite in Fe-27%Ni-0.27%C alloy changed from lath to lenticular with the variation of grain size. From the above results, it was concluded that the martensitic transformation temperature and morphology was mainly dependent upon the austenite grain size.