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The transition from a flat solid-liquid interface to a skeletal shape during BaB2O4(BBO) single crystal growth in Li2B4O7 flux is observed in real time by an optical high-temperature in-situ observation system. The movement of crystal step is also investigated. The obserwation results demonstrate that the steps propagate along and parallel to the flat interface when the crystal size is small.Nevertheless, they will ‘bend’ close to the face centre if the crystal size becomes greater. Atomic force microscopy reveals that more deposition places near the face centre give rise to the bending of advancing steps and thus the formation of a vicinal interface structure. Measurements of step velocity show that the velocity keeps nearly constant at different moments for one specific step, whereas the step on a newly formed layer advanced faster than that on a previously formed one when the crystal size is larger than 210μm or so. Thus interfacial morphological instability occurs and a skeletal interface is obtained.