A mathematical model has been proposed for describing the carbothermic nitridation of carbon/metal oxide pellet in a stream of nitrogen. Based on the model, dimensionless governing equations were derived to represent the non-isobaric system. Effects of the following five parameters on the conversion of solid C and yield of solid E have been found to be similar to that of an isobaric system: the reaction/diffusion ratio sigma(2), the reactivity ratios beta(1) and beta(2), the reactant amount ratio gamma and the Sherwood number N-Sh The reaction rate was found to be high when the value of a dimensionless viscous constant B* was large. The reaction rate for the non-isobaric system was generally found to be lower than that for the isobaric system.