Tungsten composites reinforced with 20 vol% of different particle sizes of ZrN and W powders were prepared by ball milling and spark plasma sintering. Correlations between the W and ZrN grain sizes, and the mechanical and ablation properties of the composites, were investigated. The improved mechanical properties of the ZrN/W composite containing fine ZrN and fine W grains is attributed to a W grain refinement effect and a ZrN particle strengthening effect. The mass ablation rate of the composites containing fine ZrN and coarse W grains significantly decreased, i.e., 36% lower that of the composite containing fine ZrN grains and fine W grains. The improved ablation resistance is attributed to better surface coverage of the ZrO₂ oxide layer because of uniformly dispersed fine ZrN particles, and to less oxidation and sweeping away of the W matrix during ablation through grain boundary diffusion suppression caused by the large W grain size.