Reduced activation ferritic/martensitic steels such as Eurofer 97 represent the most mature option for the fusion blanket structural material. Recently, powder metallurgy or spark plasma sintering techniques have been used to produce very high strength oxide dispersion strengthened Eurofer-ODS steels which have improved levels of creep resistance and irradiation tolerance. However, there remain concerns with these materials due to an incomplete understanding of its creep deformation mechanisms, and how the boundary condition (such as constant load or constant strain control which both widely exist in components operate at high temperature) can affect its creep deformation. It is thus timely to use in situ neutron diffraction to investigate how the addition of nanoparticle (Y2O3) affects the evolution of intergranular strain during constant load and constant strain control creep.