Purpose: To investigate the rotational load fatigue performance of different implant diameters and abutment platforms of a one-size implant-abutment (OSIA) connection system. Materials and Methods: Narrow-, regular-, and wide-diameter implants were tested with regular base (RB/WB) abutments of an OSIA system (BLX, Straumann). Wide-diameter implants (WDIs) were also tested with wide base (WB) abutments. This resulted in four test groups: (1) narrow-diameter implants (NDIs; 3.5 mm) with RB/WB abutments, (2) regular-diameter implants (RDIs; 4.0 mm) with RB/WB abutments, (3) WDIs (5.0 mm) with RB/WB abutments, and (4) WDIs (5.0 mm) with WB abutments. Each group tested five of their respective implants (n = 5). A rotational load fatigue machine applied a sinusoidally varying stress at a 45-degree angle, producing an effective bending moment of 35 Ncm at a frequency of 10 Hz in 20ºC air. The number of cycles to failure was recorded, and the results were evaluated using two-way ANOVA. Failed specimens were examined with a scanning electron microscope (SEM) to evaluate the failure mode. Pristine specimens were sectioned to examine the implant-abutment connection. Results: All specimens in three of the test groups with RB/WB abutments failed within the range of 558,750 cycles to 4,497,619 cycles, while the WDI-WB test group reached the upper limit of 5 million cycles without failure. A significant difference was found between abutment platforms (P < .001). There were no significant differences found for implant diameters (P = .857). However, with a greater implant diameter, implant fracture was less common, and the failure location was more coronal. It was consistently at the implant platform level for the abutment and at the screw neck. Conclusions: For WDIs, WB abutments exhibited superior fatigue performance compared to RB/WB abutments and would be preferred in situations of high mechanical risk. Increasing the implant diameter, when used with RB/WB abutments, did not improve fatigue performance due to the one-size prosthetic connection; however, failures were less catastrophic and coronally located, which may be advantageous in managing failures.