The (Ta, W)C cubic phase distribution plays a key role in the microstructure and mechanical properties of ultrafine WC-Co-Cr3C2-TaC cemented carbides. By integration of thermodynamic calculations and key experiments, the influence of the cubic phase distribution in ultrafine WC-10Co-0.5Cr-xTa cemented carbides was systematically investigated. A series of ultrafine grained cemented carbides were designed and fabricated through ball-milling and vacuum sintering at 1410 degrees C for 1 h. The microstructure was investigated using scanning electron microscopy (SEM). The electron backscattered diffraction (EBSD) was used to measure the orientation and size of cubic phase segregation. The results indicate that the cubic phase in the microstructure distributes more heterogeneously in the range of 0.2 to 0.7 wt% Ta addition, but finally the isolated cubic phase is homogeneously distributed with a Ta content from 0.7 to 1 wt%. Combining the thermodynamic calculation with the experiment, the mechanism for the microstructure evolution has been revealed. The mechanical properties of alloys substantially depend on the cubic phase distribution in the microstructure. A synergetic correlation between the transverse rupture strength (TRS) and Rockwell hardness was observed. The homogeneity of cubic phase can be designed and controlled effectively via the present approach.