Low-frequency Raman spectra of NaN03 have been observed as a function of temperature in a range from room temperature to above the melting point Tm. A strong central component appears and grows up with increasing temperature. Both of the width and intensity of the component show an anomaly near the phase transition point Tc. The anomalous part of the intensity shows temperature dependence as (Tc- T)in below Tc, indicating a close relation between the spectra and the order parameter in the phase transition. In terms of a spectral shape analysis using a simplified model, the origin of the central component is assigned to be a precessional free-rotation of N03 molecules. It is derived that the precessional free-rotational motion of N03 is essential for the order-disorder mechanism in the phase transition of NaNQ3.