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.