The Judd-Ofelt intensity parameters, ${\mathrm{\Omega }}_{\mathit{t}}$ (t=2,4,6) for f-f transitions of ${\mathrm{Er}}^{3+}$ ions doped in ${\mathrm{B}}_2$ ${\mathrm{O}}_3$-${\mathit{R}}_2$O (R=Na,K) glasses were determined from optical-absorption measurements, and their compositional dependence was investigated systematically. The values of ${\mathrm{\Omega }}_2$ exhibited a maximum around 25 mol % ${\mathit{R}}_2$O, while those of ${\mathrm{\Omega }}_4$ and ${\mathrm{\Omega }}_6$ decreased monotonically with an increase in ${\mathit{R}}_2$O content. The compositional dependences of ${\mathrm{\Omega }}_{4,6}$ were consistent with those of the isomer shift in ${}_{}{}^{151}{}_{}{}^{}\mathrm{Eu}$ Mössbauer spectroscopy, which gives information about the 6s-electron density of ${\mathrm{Eu}}^{3+}$ ions. The variation of ${\mathrm{\Omega }}_2$ against ${\mathit{R}}_2$O content was related to the change in asymmetry of the rare-earth ligand due to the structural mixing of borate groups, while those of ${\mathrm{\Omega }}_{4,6}$ were related to the local basicity of rare-earth sites in the glass.