The Thermal Conductivity of ZrB2 with 0–10 Vol.% ZrC Was Studied from 25 to 2000 °C. the ZrB2-ZrC Ceramics Were Prepared by Attrition Milling the Starting Powders and Adding 0.5 Wt.% C as a Sintering Aid. the Ceramics Were Hot-Pressed to Full Density at 1900 °C for 30 Min in a Flowing Ar Atmosphere. the Ceramics Contained Graphitic Carbon as a Secondary Phase, with ZrB2 and ZrC Grain Sizes of Approx. 3.5 and 1.5 Μm, Respectively. at Room Temperature, Thermal Conductivity Was 75 W/m·K for Monolithic ZrB2, Increasing to 79 W/m·K for 2.5 Vol.% ZrC, and Decreasing to 72 W/m·K for 10 Vol.% ZrC. at 2000 °C, Thermal Conductivity Was 72 W/m·K for ZrB2, Decreasing to 69 W/m·K for 10 Vol.% ZrC. Electrical Resistivity Behavior Was Similar to Thermal Conductivity, Decreasing with the Addition of ZrC Up to 2.5 Vol.% (9.9 µΩ·cm at 25 °C), Then Increasing with Further ZrC Additions. the Increase in Thermal Conductivity with ZrC Addition Up to 2.5 Vol.% Was the Result of Preferential Segregation of W from the ZrB2 Phase to the ZrC Phase. the Reduction in Thermal Conductivity with Greater ZrC Additions Was Due to the Lower Thermal Conductivity of ZrC.