The pseudo-binary systems BaB2O4-Li2B2O4 and BaB2O4-Li2O in ternay system BaO-Li2O-B2O3 have been studied by means of X-ray diffraction and thermal -analysis. A new compound 4BaB2O4 ·Li2B2O4 which is formed by peritectic reaction at 930 ± 3℃ has been discovered in the BaB2O4-Li2B2O4 system. There is a eutectic horizontal from 4BaB2O4 ·Li2B2O4 to Li2B2O4 at 797±3℃ with a eutectic point component of 79 mol% Li2B2O4. In BaB2O4-Li2O system, there also exists a compound of 4BaB2O4 ·Li2B2O4, formed by peritectic reaction. But its peritectic reaction temperature dropped to 908±3℃ with increasing amount of Li2O. Another new compound 2BaB2O4 ·3Li2O is formed by peritectic reaction at 630±3℃ and eutectic reaction occur from 2BaB2O4 ·3Li2O to Li2O at 400±3℃ and from 2BaB2O4 ·3Li2O to Li2CO3 at 612±3℃. No solid solution is observed in BaB2O4-Li2B2O4 and BaB2O4-Li2O systems.The X-ray powder diffraction pattens of 4BaB2O4 ·Li2B2O4 and 2BaB2O4 ·3Li2O have been indexed by computed programme. It is shown that the space group of 4BaB2O4·Li2B2O4 is Pmma with lattice parameters α = 13.033?, b=14.630?, c = 4.247?. There are two formula units per unit cell. The space group of 2BaB2O4 ·3Li2O is Pmmm. Lattice parameters are α = 4.814?, b = 9.897?, c=11.523?. There are two formula units per unit cell.