Using a neutron diffraction structure analysis method it is found that an order-disorder phase transition of the first kind occurs in a nonstoichiometric niobium carbide NbC, at a temperature of ~1300 K in the composition range NbCo,sl-NbCo,ss. The phase transition channel which incorporates five nonequivalent superstructural vectors belonging to three stars is determined. A carbon atom distribution function is obtained which depends on three one-component order parameters.