Yttria-stabilized zirconia (YSZ)-based engineering ceramics are manufactured by adding non-oxide additives as the typical toughening mechanism of zirconia (ZrO2). In this study, YSZ-10/20 vol% WC composite ceramics were prepared by pressureless sintering and hot pressing to view reaction pathways based on different driving forces. During the reactions, the decarburization of WC and the formation of ZrC were investigated at temperatures of ≥1500 °C. W was detected at 1700 °C, which is related to the thermodynamic stability of various chemical reactions in the ZrO2-WC binary system. The hardness of the composite ceramics decreased, whereas the fracture toughness and wear resistance increased. The phase evaluation and reaction pathways from the thermodynamic perspective derived from the YSZ-WC binary system suggest considerable potential for the future composition designs of ZrO2-based non-oxide composite ceramics.