By using X-ray diffraction, scan electron microscope, X-ray electron energy analysis and magnetic measurement, the relationship between magnetic properties and microstructure for the high intrinsic coercivity 2:17 SmCo permanent magnets during aging treatment were investigated. It is found that the alloy decomposed into a 2 : 17 phase and a 1: 5 phase as soon as the aging treatment started, and the precipitates at grain boundaries have an effect on the enhancement of coercivity. The variation of intrinsic coercivity at different aging processes can be considered as the variation of the domain wall energy difference between the two phases.