The effect of cooling rate on the morphology of lath martensite in 18%Ni maraging steel, Fe-18%Ni and Fe-24%Ni alloys was investigated in detail. The alloys were cooled from their austenite states at several different cooling rates using iced brine, water, oil, air or in furnace cooling. The main results obtained are as follows: (1) The sizes of packets and blocks in lath martensite structure are markedly changed with cooling rate. The packet size and block width decrease with increase in cooling rate. This tendency was observed in all three alloys examined. (2) Variations in M_s, M_f and the martensite-temperature curve of lath martensite in Fe-24%Ni alloy with cooling rate are hardly observable. (3) Observations of the formation process of lath martensite structure with the Greninger-Troiano heat treatment indicated that the block regions correspond to the banded region which are clustered with parallel laths, and the packet regions are formed by the growth and coalescence of parallel banded regions. (4) As the cause of the morphology change of lath martensites, it is concluded that the number of banded regions with parallel laths increases with increase in cooling rate, and hence the final block width and packet size becomes smaller.