Magnetic properties and microstructure of fully nitrided (Sm1-yCey)2Fe17Nx (y = 0, 0.20, 0.33, 0.40, 0.45, 0.50, 0.67, 0.80, 1.00) powders have been investigated. When 33 atomic% Ce is substituted for Sm, maximum energy product (BH)max changes from 17.8 MGOe to 17.6 MGOe with almost no decrease. Accordingly, performance/rare earth price ratio increases by 39.2%. According to the energy dispersive spectroscopy (EDS) analysis, Ce tends to be distributed in the REFe3/REFe2 grain boundary phase. Meanwhile, valence change of the Ce ion in (Sm1-yCey)2Fe17Nx was confirmed by XPS.