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Strontium-lead titanate semiconducting ceramics with low room temperature resistivity (ρRT) and variable NTCR-PTCR composite effects were fabricated at different sintering conditions. Both ρRT and the NTCR effect decreased at the higher heating rate or doping excess of PbO. After the heat-treatment at 950°C, the transformations from PTCR to NTCR-PTCR characteristics were observed in excess PbO-doped samples. According to the results, it is assumed that the lead vacancies (VPb″) in the grain boundary layers degenerate the conductivity of strontium-lead titanate semiconducting ceramics and the NTCR behaviors below the Curie temperature are ascribed to the electron detrapping of acceptor defects with increasing temperature.