The phase relation during cooling of molten LixLa(1-x)/3NbO3 (LLN) was investigated by the slow cooling floating zone (SCFZ) method. It was found that LLN melts incongruently into LaNbO4 and a liquid. From the results of an electron probe microanalysis (EPMA) of the SCFZ samples, the solubility limit of Li into LLN was estimated to be x = 0.29, and the distribution coefficient of Li into LLN appeared to be 0.54. The melting temperatures of LLN with various Li content were determined by differential thermal analysis. The results of the present work could be valuable for the growth of high-quality LLN single crystals.