Preparation of single crystals of LaCuOS, that is a transparent p-type semiconductor exhibiting violet-light emission, was demonstrated by the flux method using NaCl+KCl(1:1) as flux. The crystals were grown in an alumina tube inserted in an evacuated silica glass tube using the starting materials of La 2 O 2 S and Cu 2 S, as well as the flux and a small amount of K 2 S that reduces the coloration of crystals. Transparent colorless laminate crystals were obtained under the optimized crystal growth conditions; growth temperature 850°C, growth time of 72h and cooling rate 10°Ch −1 . The size of the largest colorless crystal was 3.0×2.8×0.049mm 3 . The crystal showed transmission as high as 60% and p-type electrical conductivity of 7.1×10 −4 Scm −1 at room temperature.