Abstract： Objective To compare the difference and repeatability between anterior segment optical coherence tomography (AS-OCT) and ultrasound biomicroscope (UBM) in measuring central anterior chamber depth (ACD). Methods Twenty-nine patients (32 attacked eyes and 23 unaffected fellow eyes) with acute primary angle closure glaucoma (APACG) and 41 patients (74 eyes) with chronic primary angle closure glaucoma (CPACG) were included in this study. AS-OCT and UBM were used to measure the ACD of these APACG and CPACG subjects. Pair sample t test and correlation analysis were introduced to analyze the difference and correlation of the ACD values between the 2 instruments. ACD of one complete randomly selected eye was measured 10 times by the above 2 instruments, respectively, and the repeatability of 2 instruments was analyzed. Results There was no significant difference in ACD values of attacked eyes of APACG between AS-OCT and UBM. In unaffected fellow eyes and CPACG eyes, the ACD values measured by AS-OCT were deeper than that measured by UBM[ (1.923±0.287) mm vs (1.860±0.243) mm, (2.017±0.054) mm vs (1.913±0.052) mm, both P=0.012]. Positive correlation between AS-OCT and UBM in measuring ACD was found (r=0.928, P<0.001). The coefficient of variation of ACD was 0.66% as measured by AS-OCT, and 0.82% as by UBM. Conclusions The ACD measured by AS-OCT can be deeper than that by UBM. Excellent correlation is found between AS-OCT and UBM measurements. AS-OCT appears to be more repeatable in measuring ACD than UBM.