Aim: To compare the marginal accuracy of two types of ceramics ;celtra duo and IPS Emax CAD vonlays restoring maxillary premolars.

Materials and methods: A natural tooth representing upper first premolar was prepared following ceramic onlay restorations preparation guidelines. After tooth preparation, tooth was duplicated into twenty epoxy dies using silicone mold. Epoxy dies were randomly divided into two equal groups (n= 10/group). Identical vonlays were constructed from two different materials. For Group I : vonlays were constructed from lithium disilicate blocks (IPS E.max Cad). For Group II : vonlays were constructed from zirconia reinforced lithium silicate blocks (Celtra Duo Cad). All vonlays were cemented to their corresponding epoxy dies using dual cure self adhesive resin cement (breeze). Marginal integrity was evaluated by measuring the marginal opening (MO)at three points on the margin of each surface of the vonlays by using stereomicroscope with magnification of 30X.
Results: It was found that Emax CAD vonlays recorded statistically non-significant higher marginal gap mean value (90.64 ±9.64) than Celtra duo vonlays (88.46 ±9.92). All the tested vonlays results were within the range of the clinically accepted value.
Conclusion: vonlays fabricated on premolars from Celtra Duo CAD blocks yielded comparable marginal accuracy as that obtained with IPS Emax CAD block which are clinically acceptable.
Keywords: onlay, veneer, vonlay, celtra duo, Emax Cad, marginal integrity, Cad Cam.