Statement of problem: Zirconium oxide all ceramic restorations became a popular alternative to esthetically restore missing teeth. A variety of different cements could be used for cementation. However, the retentive quality of the selected cement cannot be evaluated unless determined after being subjected to conditions simulating intraoral stresses. Aim of the study: The aim of this study was to investigate the effect of thermo-dynamic loading on retention of zirconia ceramic crowns when cemented with two different adhesive resin cements. Material and methods: Twenty intact human molars were mounted in acrylic resin blocks and prepared to receive zirconia crowns. Standardized tooth preparation was carried out by CNC milling machine to have: convergence angle of 10 degree, flat occlusal reduction and 1.2 mm deep chamfer finish line and 5 mm occluso-gingival height. Crowns were fabricated according to manufacturer's instructions using Ice zirconia blocks. Milling was carried out by CAD/CAM Laserdenta Milling machine. Teeth were divided into two main groups (10 specimens for each group) according to type of the adhesive cement used. Each group was farther subdivided into two subgroups (each of 5 Specimens TDL and Non-TDL). Milled crowns were cemented to the prepared teeth using Variolink N total etch and RelyX Ultimate self-etch adhesive resin cement. For each group, the retentive strength was determined directly after cementation and after thermo dynamic loading which clinically simulate oral conditions. Specimens were then tested for tensile bond strength directly after cementation and after thermo dynamic loading. Statistical analyses were performed using Two-way Analysis of Variance (ANOVA) was used in testing significance for the effect of cement type, thermo-dynamic loading (TDL) and their interactions on mean retention. Tukey’s post-hoc test was used for pair-wise comparison between the groups when ANOVA test was significant. Results : Regarding the effect of cement type on retention of zirconia ceramic crowns, results revealed that Variolink N cement showed statistically significantly higher mean retention value (476.8 N) than Rely X Ultimate cement (323.4 N). Regarding the effect of thermo-dynamic loading on retention of zirconia ceramic crowns, results revealed that Non-thermodynamic loading showed statistically significantly higher mean retention value(569.2 N) than thermo-dynamic loading (230.9 N).Regarding the Failure mode in retention of zirconia ceramic crowns, the analysis in the present study showed that before thermo-dynamic loading, there was a statistically significant difference between the two cements. Rely X Ultimate cement showed higher prevalence of cohesive (Tooth) failure while Variolink N showed higher prevalence of cohesive (Crown) failure. After thermo-dynamic loading, there was no statistically significant difference between the two cements which showed both combined modes of failure. The Variolink N showed (40%) cohesive failure in cement, (40%) cohesive failure in tooth and (20%) adhesive failure at tooth/cement interface. Whereas for Rely X Ultimate there was prevalently cohesive failure in cement (80%), as well as adhesive failure at tooth interface (20%). Conclusion: Within limitation of this study, the following conclusions can be drawn: • Zirconia-ceramic crowns cemented using total etch adhesive resin cement showed higher retention than those cemented using self-etch adhesive resin cement. • Thermo-dynamic loading resulted in a significant reduction in retention of zirconia crown cemented using both adhesive resin cements. Though the total etch system still had superior retention strength.