The very large enhancement of the Curie temperature in the novel ferromagnetic nitride results from the obvious change of the electronic structure compared with that of its paternal compound . Their electronic structures in both spin-polarized and non-spin-polarized states are calculated in the local spin-density functional approximation. From the band structure obtained by us, the Curie temperature of these compounds is calculated by including the effects of both the Stoner excitation and the spin fluctuation on the basis of the theory of Mohn and Wohlfarth without any adjustable parameters. The calculated enhancement of the Curie temperature for the nitride is in good agreement with the measurement. Our results concerning the enhancement of the Curie temperature for this ferromagnet upon nitrogenation suggest that the spin fluctuation is an important factor for this class of compounds, while the Stoner excitation must be involved in the calculation as well.