Thermally poled niobium borophosphate glasses in the system 0.55(0.95−y) NaPO₃+y/2 Ba(PO₃)₂+0.05Na₂B₄O₇)+0.45Nb₂O₅ were investigated for second order optical nonlinear (SON) properties. Bulk glasses were studied by Raman spectroscopy, thermal analysis, optical and dielectric measurements. The sodium to barium substitution does not lead to significant changes in optical properties, crystallization of glasses and coordination environment of polarizable niobium atoms. However, the ionic conductivity decreases drastically with the increase of barium concentration. Secondary ion mass spectroscopy has been used to determine the element distribution in the anode layer of the thermally poled glasses. The second order optical susceptibilities gradually decrease from χ⁽²⁾=2.8 pm/V to zero with the increase of barium content. Using a simple electrical model, variations of nonlinear optical responses have been correlated with dielectric characteristics.