Purpose: to study the effect of different implant abutment sizes on fracture resistance of two ceramic monolithic crown systems. Material and Methods: Research Ethics committee approval of faculty of Dental Medicine for Girls was obtained (REC18-065). Forty (40) Titanium implant supported abutments were fabricated and divided into2 groups (n=20 each) according to abutment diameter; Group (1): implant abutment diameter 4mm and Group (2): implant supported abutment diameter 4.5mm. Samples of each group were divided into 2 sub-groups (n=10 each) according to type of monolithic ceramic crowns. Subgroup(A): zirconia ceramic crowns, Subgroup(B): vita suprinity crowns. All crowns were cemented to implant abutments by Rely X resin cement, then subjected to thermal cycling and seated on a universal testing machine and subjected to fracture resistance test. Data were statistically analyzed. Results: It was found that the highest mean ± SD values of fracture resistance were recorded for large diameter Zirconia subgroup (1032.2±174.78 N) followed by large diameter Vita suprinity subgroup mean ± SD values (935.7±83.71 N) then small diameter Zirconia subgroup (930.3±69.53 N), meanwhile the lowest mean ± SD values were recorded with small diameter Vita suprinity (815.9±101.52 N). Conclusion: The mean fracture values of zirconia were higher than that of vita suprinity crowns, while the crowns were supported on large implant abutment diameter had higher fracture resistance than that supported on smaller implant abutment diameter.