The oxidation behaviors of fused zirconium diboride and chemosynthetic zirconium diboride as well as morphology and composition of their oxidation products were researched by FESEM-EDS and XRD. The two kinds of zirconium diboride were heated at 700 ℃, 900 ℃, 1 100 ℃ and 1 300 ℃ for 3 h in air, respectively. The results show that ZrO2 and B2O3(l) are generated from the chemosynthetic zirconium diboride oxidized at 700 ℃ for 3 h or the fused zirconium diboride oxidized at 800 ℃ for 24 h; B2O3(l) dissolves into water and then H3BO3 crystallizes.