Mechanical properties of nano/micro-silica particles bidispersed epoxy composites were investigated based on experimental results. The composite specimens varied with different compositions of nano and micro-silica particles (240 nm and 1.56$m) were prepared with the constant volume fraction, 0.30. The thermo-viscoelastic properties for the composites and the neat epoxy measured in the temperature ranges from 123 K to 523 K and compared to theoretical results according to Lewis and Nielsen’s law with the maximum particle packing given by Ouchiyama and Tanaka’s model. In addition, fragility derived from the thermo-viscoelasticity measurements was used to characterize the strength and fracture toughness of the composites. From results, we found that the thermo-viscoelasticity of the composite was dependent on nano and micro-particles packing, and its strength and fracture toughness were effectively evaluated by fragility.