The microstructure of twinning as well as the phase boundary between 1∶5H and 2∶17R phase in Fe-rich Sm2Co17-type magnets was characterized at atomic scale using nanobeam diffraction and high-resolution STEM-HAADF imaging,and the reason for the dramatic increase of coercivity during slow cooling was investigated based on the microchemistry analysis.The twinning relationship in the 2∶17R phase originates from ordered substitution of Sm atoms by Co-Co atomic pairs on every three (3033)and (30(3)3) planes,leading to formation of two corresponding equivalent twin variants.The basal plane of the 2∶17R phase,the 1∶3R platelet phase across the 2∶17R cell and the 1∶5H cell boundary phase between two adjacent 2∶17R cells all can act as effective twin boundary,The cell boundary phase is precipitated along the pyramidal habit plane,and a fully coherent phase boundary (PB) is formed be-tween the 1∶5H and 2∶17R phases with the orientation relationship to be PB//(11(2)1)1∶5H//(10(1)1)2∶17R.The phase boundary may either be parallel to or intersect with the pyramidal planes occupied by Co-Co atomic pairs.The substantial increase of coercivity during slow cooling is ascribed to the development of large gradient of the elements concentration within the cell boundary phase,resulting in large gradient of domain wall energy,and thus the pinning strength of the cell boundary phase against magnetic domain wall motion is significantly enhanced.