Objectives　To provide anatomic data for mastoidectomy through the posterior transpetrosal approach.    Methods　The mastoidectomy was simulated in 25 cadaveric specimens. Electric drill was used to move the superficial bone of the mastoid to expose the underlying structures. The sigmoid sinus, superior petrosal sinus, dura mater of middle cranial fossa, posterior cranial fossa, bony semicircular canals and mastoid segment of the facial nerve were exposed and skeletonized. Variations of the structures in the mastoid were observed and important data were collected.　Results　Mastoid bone can be divided into three layers including the superficial cortical bone, the mastoid air cell and the Ivory bone encasing the sigmoid sinus, the facial nerve and the labyrinth. The minimal distances from the superficial mastoid to the sigmoid sinus, the junction of the transverse sinus, sigmoid sinus and the superior petrosal sinus, the top point of the eminence of the horizontal semicircular canal，the middle part of the mastoid segment, the digastric ridge was  （7.28±2.14）mm,（14.22±2.74）mm,（16.56±2.10）mm,（13.31±1.87）mm,（11.58±1.60）mm separately. The variations of the sigmoid sinus included the enlargement of the sinus, the far-advanced sigmoid sinus and high jugular bulb. No variations of the mastoid segment of facial nerve and the bony semicircular canals were observed.　Conclusions　Be familiar with the anatomic characteristics of the mastoid can make the mastoidectomy more safe and efficient.