Ultracoarse grain WC-Co cemented carbides are widely used as tool materials for service under extreme conditions. The serving life was affected greatly by the toughness and the deformation mechanisms. In this work, the ultracoarse grained cemented carbides were prepared by sintering the in situ synthesized nanocrystalline and coarse WC powders. Using the designed samples, the indentation-induced deformation was obtained by a Vickers hardness tester with a load of 30kg. The stress and strain distributions under and around the indenter were predicted by the finite element simulations. The scanning electron microscopy and electron backscatter diffraction were used to study the microstructure morphology and WC crystal orientations. The hardness and elastic modulus of the single WC crystal in the deformed zone were measured by the nanoindentation technology. The study proposed that slipping in the WC grains play a significant role in the deformation behavior and the toughness of the ultracoarse grain cemented carbides.