Transmission electron microscopy study of pressureless sintered ZrB2-20 vol% SiC composites with WC (5-10 vol%) additions was carried out in this work. Two independent chemical reactions in which the ZrO2 impurities were removed from ZrB2 grain surface were confirmed. Three new formed phases, (W, Zr)(ss)B, (Zr, W)(ss)C, and (W, Zr)(ss)Si-2, apart from the plate-like ZrB2 and SiC grains, were identified in the samples. Microstructure observations strongly suggest that the liquid phases in terms of (W, Zr)(ss)B and (W, Zr)(ss)Si-2, appear in this system during the sintering process below 2200 degrees C, and the former could well wet the ZrB2 grain boundaries. The mass transport process between ZrB2 and (W, Zr)(ss)B was also confirmed by scanning-transmission electron microscope analysis. The above results supports that the densification at 2200 degrees C is assisted by liquid phase and the elongation of ZrB2 platelets resulted from the Ostwald ripening during the dissolution-diffusion-precipitation process.