The R-curve for mullite-molybdenum (32 vol%) composites, which were obtained at 1650degreesC under reducing conditions with three different Mo average grain sizes (1.5, 3, and 9 mum), was estimated by the indentation-strength method and compared with that monolithic mullite obtained under similar conditions. The composites material exhibited rising R-curve behavior. The composite with larger grain size, however, displayed better damage tolerance and higher resistance to crack growth. Microscopic observation of the crack path revealed, in the composites, the systematic presence of dispersoids acting as bridging sites in the crack wake. Therefore, the increased fracture toughness of these ceramic-matrix composites with adherent ductile phase can be attributed to clamping forces applied by metal ligaments that bridge the crack faces behind the crack front. These clamping forces retard the crack from opening as an external stress is applied. It was inferred that this superior performance of the larger Mo particle size composite can be attributed mainly to more effective bridging of the metal grains. Because of this, a higher applied stress intensity will be required to propagate the crack tip. These results suggest that the rising R-curve should be proportional to the metal grain size, since the grain bridging area is proportional to the metal grain size.