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Neutron di raction experiments were performed to clarify the nature of the ferroelectric phase transition of 18O isotope exchanged SrTiO3. We found clear evidence of the change of the Bragg intensity at the phase transition temperature; presumably the symmetry is orthorhombic below the transition temperature. The preliminary structure analysis in the ferroelectric phase indicates that the displacement pattern is interpreted by the combination of R25 mode rotation and the so-called Slater mode condensation of the TiO6 octahedron.