Large YAlO3 : Nd single crystals doped with neodymium were grown by the Czochralski method. The crystal and twin structures as well as the optical properties of grown crystals were investigated. It has been shown that the twins are formed during growth process in the regions of rapid crystallization. The nucleation followed by crystallization of the melt in twin state depends upon orientation of the crystal-melt interface with respect to twinning planes.