Tetragonal PbTiO3-BiFeO3 exhibits a strong negative thermal expansion in the PbTiO3-based ferroelectrics which consist of one branch in the family of negative thermal expansion materials.Its strong negative thermal expansion is much weakened, and then unusually transforms into positive thermal expansion as the particle size is slightly reduced.This transformation is a new phenomenon in the NTE materials.The detailed structure, temperature dependence of unit cell volume, and lattice dynamics of PbTiO3-BiFeO3 samples were studied by means of high-energy synchrotron powder diffraction and Raman spectroscopy.Such unusual transformation from strong negative to positive thermal expansion is highly associated with ferroelectricity weakening.An interesting zero thermal expansion is achieved in a wide temperature range (30~500 ℃) by adjusting particle size due to the negative-to-positive transformation character.Present study provides a useful method to control the negative thermal expansion not only for ferroelectrics but also for those functional materials such as magnetics and superconductors.