It is very interesting that the KBe₂BO₃F₂ (KBBF) crystals are grown both by flux and hydrothermal method. In this paper, defects of KBBF crystals grown by these two methods were examined by the optical microscope. The main defects of flux grown KBBF crystals were etch pits and twin boundaries, which were also observed in the hydrothermally grown ones. The composition and morphologies of these defects were analyzed. Some formation mechanisms and the methods of eliminating these defects were also discussed. While the main defect for the hydrothermally grown KBBF is inhomogeneous with two structures R32 and R-3c mixing that is never discovered in the flux grown ones. It is very difficult to overcome up to date. Up to now the high power sixth harmonic generation of Nd:YAG laser at 177.3nm have all been produced by the flux grown KBBF crystals and we suspect that the hydrothermal method is not practical for KBBF crystal growth and applications.   

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