We prepared niobium carbide clusters, Nb(n)C(m), in the gas phase by a double laser ablation technique. A photoionization efficiency was measured as a function of the wavelength of an ionization laser to determine ionization energies (E(i)'s) of Nb(n)C(m) (n = 3-10, m = 0-7). The E(i)'s of Nb(4)C(4) and Nb(5)C(3) are found to be the lowest in the clusters studied. When the experimental E(i)'s are compared with the E(i)'s estimated by the density functional calculation previously reported (Harris, H.; Dance, I. J. Phys. Chem. A 2001, 105, 3340-3358), we determined Nb(5)C(3) to have a similar 2 x 2 x 2 cubic structure as Nb(4)C(4). We also found that the E(i)'s of carbon-rich clusters, Nb(n)C(m) (n < or = m), tend to be higher (> 5 eV) than the E(i)'s of the niobium-rich clusters. The high E(i)'s are due to the structure of the carbon-rich clusters: carbon-carbon bonding is preferred when the number of carbon atoms exceeds the number of metal atoms. The structure was also discussed in relation to the reactivity of cluster ions with a hydrogen molecule.