We aimed to explore whether the cuff/arm (C/A) circumference ratio within the suggested range (> 80%) affects the accuracy of mercury cuff blood pressure (BP) measurement (cuff BP) using intrabrachial BP (IABP) as a reference.A total of 253 patients aged 62.42 ± 9.70 years were included. After coronary angiography, the catheter in the right arm was gradually withdrawn toward the cubital fossa, and the IABP was continuously recorded. The cuff BP of the right arm was measured based on the artery blood flow using a special method similar to the traditional mercury method. The cuff was replaced using another C/A ratio after one minute, and the test was performed again. We used three different cuffs for each participant to meet the C/A ratios of 80%-84%, 85%-89%, and 90%-100%. We calculated the percentage deviation degree (DD) between the cuff BP and IABP values: DD = difference/IABP × 100%. The agreement between the values was evaluated using the Bland-Altman method.The IABP values were 138.52 ± 16.89/79.67 ± 9.81 mmHg. The DD of the systolic BP (SBP), with a ratio of 80%-84% (3.06%), was the smallest. The DD of the diastolic BP (DBP) was lowest at a ratio of 85%-89% (2.47%). Men and women had the lowest DD of the SBP at a C/A ratio of 80%-84% and the lowest DD of the DBP at a C/A ratio of 85%-89%. Regardless of whether the participants had coronary heart disease, the DD of the SBP at a C/A ratio of 80%-84% was the lowest, and the DD of the DBP at a C/A ratio of 85%-89% was the lowest.Even in the suggested range of > 80%, when the C/A ratio was 80%-84%, the difference in the SBP between the cuff and IABP was the lowest, but when the C/A ratio was 85%-89%, the difference in the DBP was the lowest.