Abstract: Multiphase Mo-12Si-8.5B alloy is a promising high-temperature structural material. In order to further simultaneously improve the strength and toughness of the Mo-12Si-8.5B alloy, the method of strengthening and toughening the bimodal grain size Mo-12Si-8.5B alloy with adding nano-ZrO₂ (Y₂O₃) particles was put out. Nanometer Mo-ZrO₂ (Y₂O₃) composite powders were successfully prepared by sol-gel and high-temperature hydrogen reduction method, and a series of Mo-12Si-8.5B-ZrO₂ (Y₂O₃) composites with a bimodal grain size distribution were fabricated via spark plasma sintering (SPS) using nanometer Mo-ZrO₂ (Y₂O₃) and micrometer Mo powders as raw materials. The results show that the particle size of Mo powders and the relative density of the sintered body decrease with the increase of the ZrO₂ (Y₂O₃) content. When the ZrO₂ (Y₂O₃) content is less than 2.5wt%, the relative density is above 98.1%. As the content of ZrO₂ (Y₂O₃) are 1.5wt% and 2.5wt%, the composites exhibit the high hardness (9.76-9.98 GPa), flexural strength (672-678 MPa) and fracture toughness (12.68-12.82 MPa·m^1/2). Grain refinement of the Mo, grain boundary strengthening of the nanometer/micrometer Mo grains and the second-phase strengthening of the nano-ZrO₂(Y₂O₃) particles attribute to the increase of the hardness and flexural strength. The coarse grain Mo and nanometer ZrO₂ (Y₂O₃) in the composites contribute to the improvement of the fracture toughness. The toughening mechanisms of the Mo-12Si-8.5B-ZrO₂ (Y₂O₃) composites are crack deflection and crack bridging.