The phase transitions of LiKSO4 have been investigated by means of X-ray powder diffraction and thermal analysis above room temperature. It is established that the crystal structure of LiKSO4 between the melting point and 675℃ is isostructural with the high temperature phase of α-K2SO4 and its space group is probably P63/mmc. Cooling below Ti=675℃ , there appears an incommensurately modulated structure analogous to the incommensurate phase in K2WO4, K2MoO4, etc. The parameter κ changes with temperature from 0.492(640℃ ) to 0.500(470℃ ). At 470℃ , I→C phase transition (lock-in) takes place, after which the crystal structure becomes the superstructure lattice of the room temperature phase. At 439℃ the superstructure disappears yielding to the room temperature phase (P63).