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 a¯=−1.99×10−5∕°C for pure PbTiO3 to a¯=−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.