The author reports 42 patients (56 eyes) of glaucoma performed trabeculectomy under operating microscope from August 1991 to April 1993. After operation the conjunctival filtration blebs were observed in details. The formation rate of ideal blebs was 80.3%. The success rate of operation was 87.5%. The author discusses the mechanisms of lowered intraocular pressure in trabeculectomy and the relation between the type of filtration bleb and intraocular pressure. The author believes that the formation of functional subconjunctival filtration blebs is the key to successful operation. The management of non-functional filtration blebs is presented in this paper.