The aim of this in-vitro study is to assess the effect of two different priming agents on the shear bond strength (SBS) of composite resin used to repair zirconia compared to sandblasted zirconia.-Twenty four zirconia specimens (12 ×10 ×3 mm) were sliced from presintered zirconia blank using Isomet 98 machine then the specimens were sintered according to the manufacturer instructions.-The specimens were randomly divided into three groups according to the surface treatment performed; each group was 8 specimens as follow:Group (1) (control group): The specimens were subjected to sandblasting with 110μm Al2O3 particles.Group (2) (RelyX group): The specimens were subjected to RelyX Ceramic Primer.Group (3) (Monobond Plus group): The specimens were subjected to Monobond Plus primer.-The specimens of group1 (control group) were subjected to airborne-particle abrasion with 110 μm Al2O3 particles from a distance of 10 mm at a pressure of 2.8 bar for 15 seconds.-The specimens of group2 were treated with RelyX Ceramic Primer with a clean bonding brush in a homogenous rubbing motion and left for 1 minute then gently air dried for 5 seconds.-The specimens of group3 were treated with Monobond plus primer with a clean bonding brush in a homogenous rubbing motion and left for 1 minute then gently air dried for 5 seconds.-The Te-Econom universal adhesive was applied on all specimens, air dried gently for 5 seconds and light cured for 30 seconds. The adhesion procedures were performed according to the manufacturer’s recommendations. Each specimen received two composite resin cylinders.-Thin plastic translucent tubes with 4mm internal diameter and 3 mm height were used to produce standard cylindrical composite resin and the Te-Econom plus composite resin was applied incrementally into the tube, each layer was light-polymerized for 40s then the tubes were gently cut with a sculpt blade and removed with tweezers and the composite resin cylinders were light-polymerized again for 40 s.-The three groups of samples were stored for 24 hours in an incubator at 37°C and 100% humidity, followed by thermal cycling (500 cycles at 5ºC and 55ºC) 20 seconds dwell time at each bath; 5 seconds of transition.-Finally, all samples were tested by the Universal testing machine with 500 N load cell and speed of 0.5mm/min, to evaluate the SBS of resin composite to repaired zirconia.The specimens were loaded until debonding in shear test. Shear strength is calculated through the equation: Ƭ= F (force) / A (area), where T is the shear bond strength value in MPa (1 MPa = 1 N/mm2). The interfacial area (A) was 12.57 mm2 (A = πr2, where πis a constant = 3.1416 and r is the radius of the composite cylinders = 2 mm).-Two representative specimens were randomly selected from each group to be examined using scanning electron microscope x1000 magnification to characterize the surface morphology of each surface treatment method.Results:-The sandblasted specimens of group (1) showed the highest mean shear bond strength with a significant difference while the specimens of group (2) (RelyX group) showed higher mean shear bond strength than that of group (3) (Monobond Plus group) with non-significant difference between them.1--The SEM showed that the sandblasted zirconia had the roughest surface with multiple and deep micro-irregularities. While for zirconia specimens treated with RelyX (group 2) and Monobond Plus (group 3) there were mild and shallow irregularities.