The sinus tympani is subject to great variability in size, shape and posterior extent. A heavy compact bony zone, especially in the posterior portion and the narrow space between the facial nerve and posterior semicircular canal is the limitation of surgical approach. The facial recess should be opened, creating a wide connection between the mesotympanum and mastoid in the intact canal wall tympanoplasty with mastoidectomy. The surgically created limits of the facial recess are the facial nerve medially, the chorda tympani laterally and the bone adjacent to the incus superiorly. Using adult Korean's thirty-five temporal bones, the author measured the osteologic relationship in the posterior tympanum, especially sinus tympani and facial recess. And the following result were obtained ; 1) The average distance from the anterior end of the pyramidal eminence (1) to the edge of the sinus tympani posteriorly was 2.54 (1.05-5.40) mm. (2) to the maximum posterior extent was 3.22(1.25-7.45) mm. (3) to the maximum cephaled extent was 0.67(0.40-1.75) mm. 2) The boundary of the sinus tympani was 82.9% from the lower margin oval window to the upper margin round window niche. 3) The deepest part of the sinus tympani was 62.9% in the midportion, between the ponticulus and subiculum. 4) The oblique dimension from the fossa incudis above to the hypotympanum below was 8.13(7.90-9.55) mm and the transverse dimension midway between the oval window above and round window below was 3.00(2.85-3.45) mm and the transverse dimension at the level of the fossa incudis was 1.81(1.40-2.15) mm. 5) The facial nerve dehiscence was 14.3%. 6) Anterior-posterior diameter of the footplate was 2.98(2.85-3.05) mm and the average distance from the footplate to the cochleariform process was 1.42(1.30-1.55) mm and to the round window niche was 1.85(1.45-2.10) mm.