Temperature dependence of Raman scattering spectra of thiourea crystal was studied in detail across the incommensurate phase transition. The anomalous temperature dependence of band splitting of the $NH_2$ stretching bands at around $3200cm^{-1}$ and $3400cm^{-1}$ was observed to deviate from the existing theoretical predictions of $\Delta\nu(or\Delta\nu^2)\infty(T_I-T)^{0.5}$. This $NH_2$ band may be speculated to be more susceptible to the ferroelectric order parameter than the incommensurate order parameter.
