In this paper the defects of KBe2BO3F2 (KBBF) crystals grown by the spontaneous nucleation flux method have been studied in detail. The main defects including inclusion, dislocation and twins were characterized by X-ray diffraction, polarized optical microscope and scanning electron microscope. Some reasons are proposed for the formation of these defects and approaches to reduce the defects which are helpful for yielding high quality KBBF crystals. (C) 2013 Elsevier B.V. All rights reserved.