Sizeable crystals of KBe2BO3F2 (KBBF) were successfully grown by the hydrothermal method, but their conversion efficiency of second harmonic generation (SHG) was much lower than that of the flux grown crystals. In order to find the reason, extensive studies on defects in hydrothermal-grown KBBF crystals were carried out for the first time. Dislocations, grain boundaries and twins were revealed and observed on the (0 0 1) plane of hydrothermal-grown KBBF crystals by chemical etching and microscopy. X-ray rocking curves indicate the hydrothermal-grown crystals are of poor crystalline quality. Moreover, powder X-ray diffraction measurements show some small unidentified peaks for hydrothermal-grown KBBF crystals. Powder SHG intensities of hydrothermal-grown KBBF crystals were found to be several times lower than those of the flux grown ones. These indicate the hydrothermal-grown KBBF crystals may contain structural defects. (C) 2010 Elsevier B.V. All rights reserved.