It has been found that in Fe-Ni alloys, the direct lattice transformation from γ to α without their concentration change takes place when the alloy is supercooled to some fixed temperatures which depend on Ni content of the alloys. β phase in Cu-Al, Cu-Sn alloys and γ phase in Fe-C alloy transform into intermediate phase during quenching while γ phase in Cu-Sn alloy, β phase in Cu-Be alloy is supercooled to room temperature without any change by quenching. These different behaviors of the eutectoid alloys are attributed to the different types of the diagrams which enable the direct lattice transformation above mentioned or not. The martensite and the intermediate phase having martensitic structure thus formed are the metastable α phase in each alloys.