Purpose: The aim of this study was to evaluate the effect of two types of finish line designs on the marginal accuracy of monolithic zirconia crowns after thermo-mechanical fatigue. Materials and Methods: two human first molars were prepared and duplicated using epoxy resin to produce twenty dies. Accordingly, a total of 20 monolithic zirconia crowns (N=20) were designed and constructed using CAD/ CAM machine. The crowns were divided into two groups according to the finish line design, feather edge (FE) group (n=10) and deep chamfer (DC) group (n=10). All crowns were adhesively bonded to their corresponding epoxy resin dies by using dual cure self-adhesive resin cement and subjected to thermo-mechanical fatigue simulating 3 months of clinical situations then vertical marginal gap was measured using digital microscope. Results: Statistical analysis showed that deep chamfer margin design recorded statistically significant higher marginal gap mean value (39.6383 ± 5.00193 μm) than feather edge design (32.2616 ± 2.60053μm). Conclusion: marginal accuracy of monolithic zirconia constructed with feather edge margins is better than that with deep chamfer margin designs