A new phase transition is found in a LiNaCO3 crystal at 175 degrees C. The structure of the high temperature hexagonal phase (b-phase) was determined using X-ray single crystal diffraction. The low symmetry phase (alpha-phase) is triclinic and can be described as distorted hexagonal phase. The structure of triclinic phase at room temperature was refined using X-ray powder diffraction data and the Rietveld method. The probable transformations of crystal structure at higher temperatures are discussed.