Abstract The pseudo-binary systems BaB2O4-Na2O and BaB2O4-Na2B2O4 in the ternary system BaO-Na2O-B2O3 have been studied by means of thermal analysis and X-ray diffraction. The system BaB2O4-Na2B2O4 is a eutectic one and it's eutectic reaction occurs at 826 ±3℃. A new compound BaB2O4·Na2O which melts congruently at 846 ± 3℃ has been discovered in the BaB2O4-Na2O system. There exist eutectie horizontals from BaB2O4 to BaB2O4·Na2 at 755 ±3℃ and from BaB2O4·Na2O to Na2O at 573 ± 3℃. According to the obtained phase diagrams of the mentioned binary systems, the single crystals of BaB2O4 low temperature phase with dimensions of 2×4×6 mm3 and 2×4×8 mm3 have been grown by the Czochralski method from the melt containing 15 mol% Na2O or 13 mol% Na2B2O4 as flux.