Hagag Abd El Kader Mohammed¹, Essam El Saeid Al-Wakeel², Mustafa Kamal Mohammed Yousef³ and Abeer Abd El-Salam El-Fallal⁴. ¹Demonstrator1Assistant lecturer, Dental Biomaterials, Faculty of Dentistry, Mansaura University ²Professor of Dental Biomaterials, Faculty of Dentistry, Mansaura University ³Professor of Solid State Physics (Metal Physics),Faculty of Science, Mansoura University ⁴Professor and chairwoman of Dental Biomaterials,Faculty of Dentistry, Mansoura University Abstract: Objectives:This study was conducted to evaluate the effect of sintering time and thermocycling on crystalline structure and fracture toughness of zirconium dioxide core materials(Y-TZP) (ICE Zirkon, ZirkonZahn, Italy). Methods:Forty zirconia discs were divided into two groups. Discs of the first group were sintered in sintering oven (ZirkonZahn) at 20–1500 ◦C temperature using rise time of 3hand kept at 1500 ◦C for 2h. Discs of the second group were sintered at 20–1500 ◦C using risetime of 1h 40 min and kept at 1500 ◦C for 1h. Half of the discs (n = 20) from both groupswere thermocycled in distilled water for 5000 cycles (5–55 ◦C). X-ray diffractometer was used for evaluation of crystalline structure of tested groups. For evaluation of fracture toughness 20 rectangular discs were used. V-shaped notch was created with a diamond disc. The ratio between notch depth and specimen thickness is 0.5. The discs were loaded until fracture in a four-point bending test at across-head speed of 1 mm min.The data were analyzed using Two-way ANOVA followed by the least significance (LSD) post hoc analysis for inter group comparison. Significance was considered when P value ≤ 0.05.Results: All the tested discs exhibited tetragonal structure. For fracture toughness, it was found that the highest mean fracture toughness value was for control group with normal sintering time and without thermocycling (11.3 ± 1.48 MPa.m1/2) while the lowest value was for zirconia sintered at short sintering time then thermocycled (9.22± 0.68 MPa.m1/2 ). The results revealed no significant differences between tested groups (P > 0.05). Results:All the tested discs exhibited tetragonal structure. For fracture toughness, it was found that the highest mean fracture toughness value was for control group with normal sintering time and without thermocycling (11.3 ± 1.48 MPa.m1/2) while the lowest value was for zirconia sintered at short sintering time then thermocycled (9.22± 0.68 MPa.m1/2 ). The results revealed no significant differences between tested groups (P > 0.05). Conclusions:Variation in sintering time from1.6h to 3.0h did not affect structure and fracture toughness of Y-TZP zirconia. Thermocycling of zirconia sintered at normal sintering time has no apparent detrimental effect on their properties. Keywords:zirconium dioxide; sintering time; thermocycling; X-ray diffraction; fracture toughness