Joint strengthening of oxide dispersion strengthened (ODS) martensitic steel has been attained by a transient liquid phase (TLP) bonding utilizing a newly developed ODS insert foil. The ODS insert foil (Fe-9Cr-2W-0.2Ti-0.35Y 2O3-0.5C-3B-2Si) was fabricated using mechanical alloying and a spark plasma sintering method. Compared to conventional TLP bonding with a non-ODS insert foil (Fe-0.5C-3B-2Si), the microstructure of the melted zone consists of finer grains in the joint with the newly developed ODS insert material, and the grain size is almost one third of that in the conventional insert material. This increases the hardness by ΔHv = 100 in the region of the joints. Oxide particles that are coherent to the ferritic matrix in the ODS insert foil could be responsible for the grain refinement, which is explained in terms of enhanced nucleation of consolidation matrix at the oxide particles.