The aim of this study is to elucidate the microscopic structures above the superior fornix relation to blepharoptosis operation. Eight fixed cadavers of Korean adults were used. In six cadavers, 12 orbits were explored after removal of brain. In two cadavers, histologic sections were made. Below the levator, thick fibrous sheath was covering superior rectus. According to Whitnall's description, we called the thickened portion the "conjoint fascial sheath" (CFS) of the levator and superior rectus attached to the conjunctival fornix. CFS was located 2.5 +/- 0.2 mm (range, 2-8 mm) posterior to the fornix. It was 12.2 +/- 2.0 mm (range, 8-14 mm) anteroposterior length and 1.1 +/- 0.1 mm (range, 0.5-1.5 mm) thick. The shape was equilateral trapezoid with a longer base anteriorly. Posteriorly, it was extended from the fascia of the levator and superior rectus. Anteriorly superficial and deep extensions of CFS were continued approximately 2 mm to the superior conjunctival fornix and then 2 to 3 mm distally along and beneath the palpebral and bulbar conjunctiva. Surgeons should be aware of the presence of CFS between levator and superior rectus in performing ptosis surgery.