The structures of Pb1−xCdxTiO3 (x=0.03 and 0.06) were investigated by the x-ray Rietveld method at room temperature. It is surprising to find that the spontaneous polarization displacements of Pb/Cd and Ti atoms according to the oxygen polyhedron decrease, although the c/a ratio increases with doping-cadmium content. Cadmium substitution plays a unique role of enhancement of the negative thermal expansion in PbTiO3. The average bulk thermal expansion coefficient decreases from ā=−1.99×10−5 / °C for pure PbTiO3 to ā=−2.40×10−5 / °C for Pb0.94Cd0.06TiO3. The negative thermal expansion of PbTiO3 might be a consequence of hybridization between Pb and O atoms.