PURPOSE: To produce a high-quality potassium titanium phosphate single crystal generating no striation in the crystal.

CONSTITUTION: A mixture of starting materials for a potassium titanium phosphate single crystal with starting materials for a flux is melted and a seed crystal made of a potassium titanium phosphate single crystal is immersed in the resulting melt and grown by slowly cooling the melt to produce the objective potassium titanium phosphate single crystal. At least potassium vanadate is used as the flux but it is preferable that potassium phosphate is added to the potassium vanadate from the viewpoint of the adjustment of the viscosity of the melt and the control of the gradient of a solubility curve and the form of the single crystal.