The effect of composition on the pseudo-binary ZrCr(2)-NbCr(2), ZrCr(2)-TaCr(2) and ZrCr(2)-HfCr(2) C15 Laves phase alloys were investigated, focusing upon defect structures and room-temperature fracture toughness. The defect structures were evaluated by lattice parameter and density measurement. The room-temperature fracture toughness was assessed by Vickers indentation test. Alloying, off-stoichiometry and related defect structures were generally beneficial for improving the room-temperature fracture toughness of the C15 Laves phase. The results were discussed, based on the local atomic size argument of the most closely packed planes in the C15 Laves phase where shear deformation takes place.