Mo-based Tm2O3 composites used as neutron absorbers were synthesized by ball milling, cold isostatic pressing and sintering. The size of Mo grain was decreased rapidly in the initial stage and then kept a constant in the later stage. After ball milling for 96 h, the size of Mo grain was up to approximately 8 nm. Ball milling induced Tm2O3 to be first fined, nano-crystallized, then transformed to amorphization, and finally dissolved into Mo crystal. The supersaturated nanocrystalline solid solution of Mo (Tm, O) was formed after 96 h of ball milling. Sintering caused Tm and O atoms precipitated from Mo crystal and then formed Tm2O3 precipitates that uniformly distributed in the Mo matrix. After sintered for 12-24 h at 1400-1600 degrees C, only diffraction peaks of Tm2O3 and Mo could be observed in the XRD spectrums, which indicated that there was not a chemical reaction between Tm2O3 and Mo. The microhardness of sintered bulks increased with increasing ball-milling time, sintering temperature and time, and the chemical content of Tm2O3 in the powder mixtures. The evolutionary mechanism of the microstructural characteristics during ball milling and subsequent sintering was discussed. (C) 2016 The Society of Powder Technology Japan.