Background: The literature reports excellent diagnostic performance of coronary CTA using photon-counting detector (PCD) CT, albeit obtained using various acquisition and reconstruction protocols. Objective: To assess the diagnostic performance for detecting significant stenosis of coronary CTA performed by PCD CT with various standard resolution (SR) and ultrahigh resolution (UHR) protocols, using invasive coronary angiography (ICA) as the reference standard. Methods: This prospective study enrolled inpatients undergoing coronary CTA between October 2023 and October 2024. Participants underwent coronary CTA by PCD CT, sequentially alternating between SR (collimation:120×0.2 mm) and UHR (144×0.4 mm) modes across participants. SR examinations were reconstructed into SRnormal and SRVNCa (virtual noncalcium) image sets, both using 0.6-mm slice thickness and Bv40 kernel. UHR examinations were reconstructed into UHRnormal (0.6-mm slice thickness, Bv40) and UHRthin (0.2-mm slice thickness, Bv64) image sets. Two radiologists independently measured diameter stenoses. Final analysis included patients who underwent ICA after CTA; a cardiologist reviewed ICA images to determine the reference standard. Stenoses were considered significant at a ≥50% threshold. Results: The SR group included 61 patients (mean age, 67±9 years; 46 male, 15 female; 788 segments analyzed). The UHR group included 61 patients (67±11 years; 43 male, 18 female; 825 segments analyzed). Per-segment sensitivity, specificity, and accuracy for reader 1 were 92.9%, 89.9%, and 90.5% for SRnormal; 92.9%, 91.6%, and 91.9% for SRVNCa; 96.0%, 92.4%, and 93.0% for UHRnormal; and 100.0%, 98.6%, and 98.8% for UHRthin; and for reader 2 were 92.9%, 88.8%, and 89.6% for SRnormal; 93.5%, 92.3%, and 92.5% for SRVNCa; 96.0%, 91.6%, and 92.2% for UHRnormal; and 100.0%, 98.9%, and 99.0% for UHRthin. Significant (p<.05) differences included SRVNCa versus SRnormal for specificity for both readers and accuracy for reader 2; UHRthin versus UHRnormal for sensitivity, specificity, and accuracy for both readers; and UHRthin versus SRVNCa for sensitivity, specificity, and accuracy for both readers. Conclusion: Coronary CTA performed by PCD CT achieved high diagnostic performance in SR or UHR mode. Performance was higher for SRVNCa than SRnormal and for UHRthin than either UHRnormal or SRVNCa. Clinical Impact: The findings will help to optimize protocols for coronary CTA performed by PCD CT.