The pseudo-binary systems BaB2O4-K2O and BaB2O4-K2B2O4 have been studies by means of X-ray diffraction and thermal analysis. The system BaB2O4-K3B2O4 is a eutectic one with eutectic temperature at 850±3℃ and the eutectic composition of 45 mol% K2B2O4. In BaB2-K2O system there exist two new compounds. A new compound 5BaB2O4·3K2O melts congruently at 903±3℃. The BaB2O4-5BaB2O4 ·3K2O forms a eutetic system. Its eutetic reaction occurs at 813±3℃ and 25 mol% K2O. Another new compound Ba4K6B8O19 melts incongruently at 860±3℃. The Ba4K6B8O19-K2O also forms a eutectic system. Its eutectic reaction occurs at about 788±3℃ and 67.5 mol% K2O.