The study was devoted to the thermal stability of the phases present in chromium-less and practically carbon-less Ni27Ti2AlMoNb steel. Steel with an austenitic structure was subjected to martensitic transformations using a thermal treatment, namely cooling to below the temperature Ms (beginning of the martensitic transformation), or by plastic deformation using the TRIP effect. The cooling in liquid nitrogen gave a martensitic-austenitic structure with athermal martensite α’a. The martensite had a lenticular morphology, and its volumetric share was 51%, which is typical of duplex type steels. The 50% squeeze of the austenitic steel resulted in the formation of deformation martensite α’d with a lamellar structure (this transformation is only possible below a certain temperature Md). During the experiments, the thermal stability of the two phases, α’a and α’d, at temperatures of 450°C and 550°C was examined. The changes taking place in the steel structure when the athermal martensite and deformation martensite were annealed below and above the temperature As (beginning of the austenitic transformation) were observed using a vibrating sample magnetometer and an optical microscope.