Positron annihiation, in situ observations of heating process by using 1000 kV HVTEM as well as magnetic measurements have been used to study the coercivity mechanism. The results show that Sm2Co17 precipitates themselves are not nucleation centres for the formation of reverse domains. It seems reasonable that some structural and compositional inhomogeneous regions have a fairly low magnetic anisotropy which may act as nucleation centres.Experiments show that the maximum irreversible loss appears at 750℃ during aging from 25℃ to 900℃ for SmCo5. The magnetic anisotropy of the central Sm3+ with different near neighbour Sm3+ ion distribution is not fixed. The more the structure simitar to Sm2Co17, the more probable that Sm3+ is negatively anisotropic.