Fracture toughness tests of W-Ni-Fe sintered tungsten alloys with various volume fractions of tungsten particles were carried out under various loading rates. Fracture toughness of the sintered tungsten alloys decreased with increasing volume fraction of tungsten particles. An increase in loading rate resulted in a significant decrease in fracture toughness. From the fractographic observations, the facets of cleavage fracture of tungsten particle decreased and the facets of ductile fracture of matrix increased with an increase in fracture toughness. When the volume fraction of tungsten particle is low, a stable crack dominantly grows in the matrix phase, resulting in high toughness. On the contrary, when the volume fraction of tungsten particle is high, the matrix phase exists only in the triple points among tungsten particles. So the deformation of matrix phase is restricted and consequently an unstable crack grows in tungsten particles in cleavage fracture manner.