Statement of the problem: The use of machinable monolithic lithium disilicate (LD) ceramics has been frequently used in fixed prosthodontics with well-known marginal adaptation values. However, information about the newly developed zirconia reinforced lithium silicate glass ceramics (ZLS) regarding marginal adaptation is of concern.  Aim of the study: The aim of this study was to evaluate the vertical marginal gap of CAD\CAM lithium disilicate and zirconia reinforced lithium silicate glass ceramics before and after thermomechanical cycling. Methodology: Twenty crowns were fabricated using Cerec in-Lab CAD\CAM system, and divided into two groups (10 IPS. E.max CAD and 10 VITA SUPRINITY). The completed crowns were cemented on epoxy resin die and the vertical marginal gap was evaluated. The cemented crowns were subjected to thermomechanical cycling for 75000 cycles and the vertical marginal gap was evaluated again after cyclic loading. The collected data was statistically analyzed using Student t-test and Paired t-test and the significance level was set at P ≤ 0.05. Results: There was no statistical significant difference between the two tested groups or between measurements before and after thermomechanical cycling. Conclusion: Neither the material type nor the thermomechanical cycling has an effect on the vertical marginal gap measurements of the two tested groups.