The high pressure transition and phase equilibrium beheviour of lithium potassium sulphate (LiKSO4) were studied by an improved high pressure DTA method and X-ray diffraction in a diamond anvil cell at elevated temperatures. The P-T phase diagram was accurately determined up to 3GPa and 800℃. A high pressure modification of LiKSO4 was observed beyond 1.65 GPa. The structure was determined to be orthorhombic with a = 7.933 ?, b = 9.493 ? and c= 9.679 ?, and its thermal expansion at high pressure and the stability of the quenched sample at normal pressure were also examined.