PURPOSE:To obtain a beta-BaB2O4 single crystal of nonlinear optical crystal free from alpha substances and having superior quality by growing a BaB2O4 single crystal without using fluxes and then subjecting the above to slow cooling while the above crystal stays at a temp. in the vicinity of the phase transition temp. CONSTITUTION:Crystals are pulled up by using a liquid having a composition consisting of 55.0-78.0wt.% BaO and the balance B2O3 without using fluxes, such as Na2O and NaBO2, and then cooled slowly through the temp. range in the vicinity of the phase transition temp., e.g., from 950 down to 900 deg.C at >=10 deg.C/H cooling rate, by which beta-BaB2O4 single crystals are obtained. Further, when the above crystals are passed through the range of 950-900 deg.C at a cooling rate exceeding 10 deg.C/H, polycrystals consisting of alpha-BaB2O4 and beta-BaB2O4 are formed.