Objective To explore the microanatomy of the horizontal segment of the facial nerve (FN)in temporal bone and its adjacent structures,in order to provide microanatomical datag for the clinical works.Methods In 20 adult wet skull specimens(40 temporal bones),horizontal segment of FN and their adjacent structures in temporal bones were observed according to operational layers by simulating transmagtoid approach.Results The lenglh of the horizontal segment was[(8.85±1.01)(7.10～11.25)]mm;The diameter of the horizontal segment wag[(1.88±0.65(1.55～1.90)]mm;The angle opening towards anterior direction between horizontal segment and vertical segment wag[(115.5±6.89)(109.5～128.6)]..The ansh towards posterior direction between horizontal segment of FN and tympanic tegmen wag [(28.5±3.66)(25.8～31.5)°;The vertical distance from the cochlear form process to the horizontal segment of the FN wag[(1.89±0.58)(0.90～3.05)]mm;The vertical distance from the head of the stapes to the horizontal segment of the FN wag[(2.30±0.85)(1.97～3.11)]mm;The distance between the summit of pyramidal segment of FN and the apex of shor limb of incus wag 2.55±0.21(2.10-2.90)mm;The distance between the summit of pyramidal segment of FN and the eminence of the lateral semicircuiar canal wag[(2.86±0.31)(2.23～3.56)]mm;No branching or dislocated nerve of the horizontal segment of the FN wag found in all 40 cases.Five percent(2/40)distal part of the horizontal segment of FN tracks forward lateral to the eminence of the literal semicircular canal.Conclusion Geniculate ganglion,cochlearform process,stapes,tympanic tegmen,lateral semicircular canal,incus are important landmarks by which horizontal segment of FN might be located in manipulation of mastoid;The relations of horizontal segment of FN and its adjacent structures are complicated,compact.It will be good that the manipuliter absorb a great deal of anatomical knowledge about landmarks that can be used in locating horizontal segment of FN.