Highly modified La2O3-Y2O3-B2O3 ternary glasses were fabricated by using a levitation technique. The thermal and structural properties of (50 - x)La2O3-xY2O3-50B2O3 glasses and (60 - y)La2O3-yY2O3-40B2O3 glasses were investigated. Raman scattering spectra indicated that B atoms mainly formed isolated planar BO3 triangles similar to those of crystalline LaBO3. This process was independent of the ratio of La2O3 and Y2O3. 11B magic angle spinning nuclear magnetic resonance spectra confirmed that the BO4 units that should have disappeared in the glass with highly modified compositions remained as fragmented species. Approximately 4% of the B atoms formed BO4 in the 50La2O3-50B2O3 glass. This ratio increased with an increase in the Y2O3 content, and it reached its maximum value (15%) in the 50Y2O3-50B2O3 glass. Comparison of the electron density distribution was conducted using ab initio calculations of the LaBO3 and YBO3 crystals and indicated that more electrons localize near the atomic nuclei in the Y-O bond than in the La-O bonds. Comparison of the electron density distribution was conducted using ab initio calculations of the LaBO3 and YBO3 crystals and indicated that electrons in the Y-O bond localize near the atomic nuclei compared to those in the La-O bonds. Thus, the unconventional existence of BO4 in a highly modified glass is attributed to the increase in the level of Y3+, which causes the localization of electrons near the atomic nuclei. Thus, the ratio of BO4 and BO3 in highly modified glass can be controlled by tuning the glass content of modifier rare-earth oxides, which opens a new door to glass science.