AIM:To study the pathological changes in the inferior oblique muscles and the medial muscles of V-pattern exotropia with inferior obliqued overation. METHODS:The inferior oblique muscles and the medial muscles were cut from V-pattern exotropias with inferior obliqued overation during strabismus operation treated as the strabismus group. Cross sections were stained by HE method for observing the pathological changes with light microscopy and measuring the cross-sectional area of myofibers. The inferior oblique muscles and the medial muscles were obtained from the corneal transplant donors (6 cases 6 eyes), which were treated as the control group.RESULTS:Myofibers morphology of the inferior oblique muscles of the strabismus group were presented slightly hypertrophy, hyperplasia, thicknesses differently, arranged irregularly, stripe structure obscured, and some myofiber nucleus adduction, hyaline degeneration, and a few of collagenous fibers hyperplasia. The average cross-sectional area of myofibers was 854.1±64.8μm² in the strabismus group, whereas there was no statistical difference between the control group (719.7±80.4μm²) and the strabismus group (P>0.05). Myofibers morphology of the medial muscles of the strabismus group were also founded stripe structure obscured,with some myofiber nucleus adduction, nucleus adduction and hyaline degeneration. And it mainly expresses for the decreas of myofbers, arranged loosely and irregularly, gap increased and collagenous fibers hyperplasia. The average cross-sectional area of myofibers was 324.9±633μm², while it was greatly reduced comparing with the control group(690.8±67.3μm²,P<0.05). CONCLUSION:Both of the inferior oblique muscles and the medial muscles in V-pattern exotropia with inferior obliqued overation patients showed the remarkable pathological changes.