Statement of problem: Milling of titanium produces excellent fitting margins; however, there is concern about its marginal accuracy after porcelain application since this aspect has a direct influence on marginal fit.Purpose: The aim of the present study was to evaluate the effect of metal coping thickness namely (0.3 mm& 0.5mm) and porcelain firing cycles on the marginal accuracy of milled titanium and cast nickel Chromium alloy.Materials and method: A stainless steel die was constructed to receive full veneered metal -ceramic non anatomical crowns. Titanium copings were CAD/CAM milled (Biodenta CAD/CAM system, Swiss). Ni.Cr copings were cast using lost wax technique .Both groups were subdivided into two subgroups according to metal coping thickness (0.3mm &0.5 mm). All copings were fired with compatible porcelain (Ceramic 2 in 1) for Ti and (VITA VMK MASTER) for Ni.Cr copings. Marginal discrepancy was determined on photographs of metal copings seated on the master die at 8 predetermined locations before and after porcelain firing. Student's t-test was used to compare between the two materials and the two thicknesses. Paired t-test was used to compare between mean gap distance before and after firingResults: Statistical analysis showed no significant difference between the two groups before and after firing of porcelain (P ≤ 0.05). Results of paired t- test showed a significant decrease in the mean values of marginal discrepancy after firing of porcelain for Ti while Ni- Cr group showed a non-significant increase in mean values of marginal discrepancy (P ≤ 0.05).Conclusions: Cast nickel-chromium copings demonstrated smaller vertical marginal gaps than the CAD/CAM titanium copings. CAD/ CAM titanium copings showed a significant decrease in gap distance after firing while cast nickel-chromium copings showed non- significant increase in gap distance after porcelain firing.