Mechanically alloyed TbCu₇-type Sm_12.5Fe_87.5-xTi_x (x=0, 2.5, 5, 7.5),and their nitrides have been studied systematically by X-ray diffraction, A.C. initial susceptibility, and pulsed magnetization measurement. In this series, the volume expansion by nitriding is 5.6%~7.3%, and the increment of the Curie temperature is in the range of 210 ℃~350 ℃. With increasing Ti content, the remanence decreases linearly due to the substitution of non-magnetic Ti, and the coercivity decreases rapidly from 34.6 kA/cm (43.5 kOe) for x=0 to 14.3 kA/cm (18 kOe) for x=7.5. In the Sm_12.5Fe_87.5-xTi_xN_y series, the best magnetic properties were obtained from Sm_12.5Fe_87.5N_y (x=0) with _iH_c=34.6 kA/cm (43.5 kOe), B_r=0.75 T, and (BH)_max=113.8 kJ/m³ (10.9 MGOe).