The phase transition of thiourea under high pressure has been investigated by means of second harmonic generation (SHG), pyroelectric and Raman scattering measurements. The high pressure phase VI turns out to be centrosymmetric as the results of the SHG and pyroelectric charge measurements. At high pressure some of new Raman-active modes were observed. On the basis of these experimental results it is guessed that the centrosymmetric high pressure phase VI of thiourea is induced by instability of a zone-boundary mode of the atmospheric pressure phase V.