Objective To evaluate the long term curative effectiveness and the failure cause of the nonpenetrating trabecular surgery (NPTS) for primary open angle glaucoma (POAG) . Design Randomized clinical trial. Participants 105 patients (111 eyes) with POAG. Methods 52 patients (55 eyes) and 53 patients (56 eyes) underwent NPTS and trabeculectomy respectively. The follow-up period was 30 to 48 months (mean, 40.1?8.2 months). Main Outcome Measures Visual acuity, intraocular pressure(IOP), filtering blebs, alteration of anterior chamber angle and complications. Results Visual acuity in the NPTS group was significantly superior to that in the trabeculectomy group (?2=7.7381,P0.05, respectively). But the successful rate in the NPTS group 48 months after operation was significantly lower than that in the trabecalectomy group (?2=5.26,P