Abstract: To study the change law of bubble defects before and after the sintering of yttria-stabilized tetragonal zirconia (Y-TZP) ceramics and the influence of bubble defects on fracture toughness of materials, the flaws of 3Y-TZP ceramic before and after sintering were observed by X-ray microscope, and the variation law of defects was obtained. Then, testing the bending strength of 3Y-TZP, 4Y-TZP, and 5Y-TZP ceramic with different yttrium oxide content under the condition of natural defects or artificially introduced volume defects, and calculating the Weibull modulus and fracture toughness. The failure modes of each group were observed under SEM. The results show that sintering can repair the bubble defects of zirconia ceramics, and the repair rate is about 74.12%. When the diameter is less than 10 μm, the repair rate is up to 97.22%. When the diameter is more than 40 μm, the repair rate is only 20%. The diameter of more than 80% of bubble defects after sintering shall not exceed 20 μm. The bending strength of zirconia ceramics decreases with the increase of yttrium oxide content. The introduction of volume defects significantly reduce the bending strength of zirconia ceramics, of which 4Y-TZP ceramic has the best stability, and the strength decreased by only 8.64%. 5Y-TZP ceramic is the most sensitive to defects, and the strength decreased by 22.58%. The fracture toughness decreases with the increase of yttrium oxide content. The depth (a) and half-width (c) of the introduced volume defect will affect the fracture toughness of zirconia ceramics. The fracture toughness increases first and then decreases with the increase of a/c, reaching a peak value when a/c≈1.5.