Using ultrafine WC powders with different impurities contents, WC-10%Co ultrafine-grained cemented carbides with 5-9 mass% VC addition to the Co content were sintered and then HIPed or HIP-RQ (re-sintered with quenching after HIP). Their transverse-rupture strengths were measured, and the relationship between the stress σ_d acting on the fracture source and its source size 2a was evaluated using σ_d^-1 - a^1/2 and σ_d - A^-1/4 (= σ_d - (2a)^-1/2) correlation diagrams. Each cemented carbide had limiting strength which was constant σ_d when 2a ≦ 10 μm, regardless of 2a, in both diagrams. The limiting strength of σ_d was varied from 3.20 to 4.04 GPa, depending on the WC powder, VC addition amount, and HIP and HIP-RQ treatment. While the grinding residual stress proposal was unlikely to be the main cause for ultrafine-grained cemented carbide, the defect expansion proposal due to plastic deformation around the defect was able to explain a part of the phenomenon. It was revealed that if the stress applied to the defect varies due to interactions between defects, this could be the cause of the limiting strength phenomenon. By considering this interaction, it is possible to explain phenomena that could not be explained by the previous proposals.