Potassium lithium niobate(KLN) single crystal for a nonlinear-optic material, which changes the wavelength of lasers, has a ferroelectric tetragonal tungsten bronze structure at room temperature. It has been very hard to get single crystals of good quality due to the cracks during cooling process. In order to investigate the composition change due to the evaporation of solution during the growth, the thermogravimetric analysis was carried out. In atmospheric condition at $1000^{circ}C$ which is about $10^{circ}C$ higher than the crystal growing temperature, the weight change was negligible amount of $1.46{ imes}10^{-5}$g/($ extrm{cm}^2$hr). By using both the Pt plate as the nucleation site and the slow cooling method with temperature fluctuation, KLN single crystal of good quality of size 1 cm could be obtained. The phase transition temperature was $490^{circ}C$, which was higher than that reported by other researchers of the other composition. The optical anisotropy due to the absorption of OH-band exists in the range of IR.