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[Formula: see text] 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.