Recent development of the inelastic X-ray scattering (IXS) technique using third-generation synchrotron facilities delivers us a wide opportunity for investigations of dynamic properties of materials. In this article, several advantages of the IXS method are explained by comparing to the corresponding neutron inelastic scattering. Intelligent techniques of IXS spectrometers at ESRF, APS, and SPring-8 are reviewed. Then our recent results on the phonon dynamics of a typical liquid metal, liquid Hg, are reported with several interesting anomalies of phonon dispersion relation and diffusion. Finally the future of this technique and its application are discussed.