Purpose: To measure the loss of retinal nerve fiber layers (RNFL) of attacked eyes of acute primary angle closure (APAC) with their fellow eyes by using confocal scanning laser ophthalmoscopy (HRT). Methods: A prospective, comparative, image diagnostic study was done to patients with a single unilateral attack of APAC. The suffered eyes were performed trabeculectomy and the fellow eyes were treated with laser peripheral iridotomy. Confocal scanning laser ophthalmoscopy was carried out at 6 month post-operation. The various parameters between the attacked and the fellow eyes (control group) were compared using the Student t-test. Results: Twenty nine patients including 8 male and 21 female with the mean age 57.97 + or - 5.97 (years) were recruited. The duration of the APAC ranged from 4 to 120 hours (mean, 47.50 + or - 30.50 hours). The mean presenting lOP during the acute attack was 37.09 + or - 6.30 mmHg. The rim volume, the height variation contour and the average of RNFL demonstrated a statistically significant difference between two groups. The retinal nerve fiber layers at temporal and inferior temporal sectors tended to loss more than those at nasal and inferior nasal sectors. Conclusion: No matter how long the duration of attack of APAC lasted, the optic disc and RNFL damage were detected quantitatively by HRT Severe RNFL loss was documented in eyes that underwent a single APAC with duration of attack longer than 24 hours.