Objective　To study structures of the posterior tympanum and to provide theoretical reference for surgical approach related to the middle ear.    Methods    30 sides of 15 cadaveric heads had their mastoids outlined and the relationship between the posterior tympanum and the other structures were observed and measured.    Results    The average distances from the midpoint of pyramid segment of facial canal to the most salient point of lateral semicircular canal, the midpoint of the anterior margin of the sinus tympani, the midpoint of vestibular window, the midpoint of cochlear window, the lower end of the ponticulus, and the lower end of the pseudosubiculum were（3.21±0.41）mm,（2.79±0.25）mm,（3.97±0.37）mm,（6.48±0.53）mm, (4.85±0.39)mm, (6.21±0.57)mm,  respectively. The average distances from the midpoint of incual fossa to the top of the pyramidal eminence, the midpoint of pyramid segment of the facial canal, the midpoint of the head of stapes, the midpoint of cochlear window, the midpoint of the vestibular window, and the cochleariform process were（4.43±0.36）mm、（3.10±0.28）mm、(6.55±0.55)mm, (9.56±0.82)mm, (6.67±0.65)mm, (4.63±0.39)mm, respectively. The average depth of sinus tympani, the average short diameter of cross section, and the average long diameter of cross section were （3.10±0.27）mm,（1.05±0.10 ）mm,（1.89±0.19）mm, respectively. The average depth of posterior tympanic sinus, the average long diameter of cross section, and the short diameter of cross section were (1.15±0.10) mm, (0.83±0.29) mm, (0.63±0.51) mm, respectively. The average depth of lateral sinus tympani, the superior inferior diameter, and the anteroposterior diameter were (1.63±0.12)mm,（0.36±0.03）mm and (0.74±0.09）mm. The average diameter of lateral facial recess gas cell was (0.28±0.13) mm.    Conclusion   Posterior tympanum with fine structure which is easy to be potential regional residual lesion is narrow. It is very important to clearly understand the anatomical structure, so as to completely clear lesions and prevent the occurrence of postoperative complications.