The development of generation IV nuclear reactor requires improvements in structural and cladding materials. Oxide dispersion strengthened (ODS) steels are promising candidates because of improved creep properties. Two main families of ODS steels are known, the first one is martensitic ODS steel with a full reverse ferrite to austenite phase transformation at high temperatures. The second one is ferritic ODS steels with a ferritic matrix whatever is the temperature. Previous papers mention the existence of untransformed ferrite (UF) phase at high temperature in very particular martensitic ODS steels [S. Ukai 2009, M. Yamamoto 2010, T. Yamashiro 2016, A. Durand 2021]. UF is unexpected according to experimental observation and thermodynamic calculations of the non-strengthened materials (NR) with same chemical composition. This UF is particularly interesting because UF is a way to improve the creep resistance of martensitic grades [H. Sakasegawa 2008, A. Durand 2021] and then conciliates the benefits of both ferritic and martensitic grades. The present study is focused on the thermal stability of untransformed ferrite (UF) in a martensitic ODS steel at 10 wt% Cr. Effects of the heat treatments holding times and temperatures are studied by considering the UF fraction evolution and microstructural changes. The decrease of UF fraction is correlated with the decrease of the nano-oxides density coupled to the increase of their size. The phase transformation temperatures of ODS steels are also affected by heat treatments. Those temperatures appear as a mix of the ODS and the equivalent non-strengthened materials ones. (c) 2022 Elsevier B.V. All rights reserved.