"To compare the ultrasound biomicroscopy（UBM） parameters before and after laser iridotomy in eyes with primary angle closure(PAC). METHODS: A prospective clinical trial was carried out in 46 cases (74 eyes) with PAC included in the study. Mean age was 58.04±11.33 (range 24.0-82.0) years. Complete ophthalmic examination, gonioscopy A-scan biometry and UBM (Vu-max, Sonomed Inc., NY,USA) and Standard Achromatic Perimetry (SAP) using the 24-2 was performed (Humphrey Visual Field Analyzer-II i, Carl Zeiss Meditec Inc., Dublin, CA, USA) in all cases. Changes in mean values between baseline and follow-up were assessed using a paired Studentˊs t test. RESULTS: The mean anterior chamber angle increased from 8.02±4.61(0.10-19.60) degree to 17.66±6.39 (0.10-32.70) degree (P=0.000), angle opening distance 500(AOD 500) increased from 0.11±0.06 (0.01-0.30) to 0.23±0.07 (0.13-0.50) mm (P=0.000),mean iris thickness(IT) decreased from 0.58±0.11(0.33-0.99)mm to 0.52±0.10 (0.25-0.77)mm(P=0.000) after laser peripheral iridotomy (LPI). No statistically significant changes were found in trabecular-ciliary process distance (TCPD), iris-ciliary process distance (ICPD) and ciliary body thickness (CBT). CONCLUSION: AOD 500, iridolenticular distance (ILT)and anterior chamber angle (ACA) increases, IT and iridolenticular height (ILH) decreases after LPI in Caucasian eyes with PAC. It means that LPI relieves the pupillary block, deepens the anterior chamber, widens the ACA, decreases the anterior bowing and thickening of the iris."