Lattice thermal conductivities of zirconium carbide (ZrCx, x = 1, 0.75 and 0.5) ceramics with different carbon vacancy concentrations were calculated using a combination of first-principles calculations and the Debye-Callaway model. The Grüneisen parameters, Debye temperatures, and phonon group velocities were deduced from phonon dispersions of ZrCx determined using first-principles calculations. In addition, the effects of average atomic mass, grain size, average atomic volume and Zr isotopes on the lattice thermal conductivities of ZrCx were analyzed using phonon scattering models. The lattice thermal conductivity decreased as temperature increased for ZrC, ZrC0.75 and ZrC0.5 (Zr2C), and decreased as carbon vacancy concentration increased. Intriguingly, ZrCx can be tailored from a thermal conducting material for ZrC with high lattice thermal conductivity to a thermal insulating material for ZrC0.5 with low lattice thermal conductivity. Thus, it opens a window to tune the thermal properties of ZrCx by controlling the carbon vacancy content.