This paper presents a transmission electron microscopy study of two Fe-1.5wt.%Mo steels stabilized with 1.5wt.%SiO2 and 1.5wt.%TiO2,respectively, and compacted through spark plasma sintering. The microstructure inspections revealed that sintered FeMo+SiO2 is able to maintain a nanometric scale grained structure up to a sintering temperature of 815°C, while the steel modified with TiO2 retained its nanometric scale microstructure up to 900°C. The ultra-fine grained structure (within 100-150 nm) was also directly correlated to the grain boundary stability through systematic extinction contours survey. Local nano-welding phenomena shows the effective compacting process of the Fe-Mo powders during sintering. Residual nano-porosity was found to decorate most of the grain boundaries and the triple grain junctions in all the sintering conditions examined, although this nano-porosity accounted for values within 0.26% in volume fraction.