LiB3O5 and CsB3O5 are two excellent nonlinear optical borate crystals containing [B3O7] groups. With a difference of aikali metal ions in structure, LiB3O5 and CsB3O5 exhibit different crystallization habits. The former is an incongruent compound, which cannot crystallize from its melt; however, the latter is a congruent compound obtained by cooling its melt directly. In this work, using Raman spectroscopy and ab initio calculation, the structures of LiB3O5 and CsB3O5 melts have been investigated, and then the influence of alkali metal ions on melt structures is discussed, finally, the relationship between crystallization habits of LiB3O5 and CsB3O5 and their melts is proposed. Results suggest that the boron oxide species of LiB3O5 and CsB3O5 melts are in the form of six-membered rings B3Ø7 and B3Ø6 (Ø represents a bridging oxygen); Raman frequency of the symmetric breathing vibration of six-membered rings shifts to low frequency with the addition of BØ4 tetrahedrons in rings; the relatively large amount of BØ4 tetrahedrons is found in LiB3O5 melts. However, Cs+ ions with larger ion radius hinder the formation of BØ4 tetrahedrons, and then reduc, the BØ4/BØ3 ratio of the melt. Finally, considering the growth mechanism of LiB3O5 and CsB3O5 crystals (Wang D, Wan S M et al. 2011 Cryst. Eng. Comm. 13 5239), we propose that the amount of BØ4 tetrahedrons in melts, which is influenced by aikali metal ions, determines LiB3O5 and CsB3O5 crystallization habits, therefore, and suggest the reduction of BØ4 tetrahedrons in melts is an effective way to crystallize LiB3O5.