Intrinsic SrTiO₃ is a quantum paraelectric,but moderately Bi-doped SrTiO₃ exhibits dielectric frequency dispersion similar to relaxor ferroelectrics. In this paper,detailed electron microscopic studies of the microstructures of Bi-doped SrTiO₃ samples were presented. It was found that the Sr sites were replaced by off-central Bi,resulting in tensile strain in the strontium titanate (STO) lattice. In the Bi-doped SrTiO₃ samples,the valence of titanium mainly showed the Ti${}^{4+}$ characteristic. According to the dielectric behavior and microstructure analysis,the polar nanoregions (PNRs) composed of strained SrTiO₃ nanoclusters should be responsible for the ferroelectric relaxor behavior in samples with moderate Bi content.