By using the X-ray diffraction profile-fitting technique, the- microstructures of Sm, Mn-modified PbTiO3 piezoelectric ceramic discs, including ferroelectric domain sizes, microstrains, and their variations with the poling strength have been quantitatively investigated. The results manifest that the modified PbTiO3 ceramics contain a high density of domain walls due to the presence of finely-divided coherent domain structures (tens of nanometres in dimension). The poling treament can evidently influence the domain-size distribution, with a more homogeneous microstructure being developed; however, it simultaneously causes high anisotropic microstrains within the structure which, together with the high density of domain walls, is expected to be responsible for the unusual high electromechanical coupling properties possessed by this material.