Background: Incarceration of the inferior oblique muscle (IO ) branch of the o culomotor nerve may occur in cases of orbital floor trapdoor fracture. Cases: Tw o orbital floor trapdoor fracture cases, with lesions located just outside of th e inferior rectus muscle but without its incarceration, were examined preand pos toperatively for visual acuity, intraocular details, the nine diagnostic ocular positions of gaze, binocular single vision field with the Hess chart, and by com puted tomography (CT). One case was also examined by magnetic resonance imaging (MRI: T1-weighted images). A forced duction test was conducted intraoperatively . Observations: Each case presented good visual acuity and neither globe showed any injury. Motility disturbance of the IO was shown in each case by binocular s ingle vision field testing and the Hess chart. The possibility of the incarcerat ion of the IO branch of the oculomotor nerve, which runs from the incarcerated l esion to the superior belly of the IO, in an orbital floor trapdoor fracture was shown on CT and MRI. Intraoperative forced duction testing revealed a restricti on due to the incarceration of the connective tissue septa. Conclusions: As infe rred from the CT andMRI analyses conducted in this study, IO palsy may be one of the causes of ocular motility disturbance of the IO in an orbital floor trapdoo r fracture, in addition to the ocularmotility disturbance due to the connective tissue septa. Background: Incarceration of the inferior oblique muscle (IO) branch of the o culomotor nerve may occur in cases of orbital floor trapdoor fracture. Cases: Tw o orbital floor trapdoor fracture cases, with lesions located just outside of th e inferior rectus muscle but without its incarceration, were examined preand pos toperatively for visual acuity, intraocular details, the nine diagnostic ocular positions of gaze, binocular single vision field with the Hess chart, and by com puted tomography (CT). One case was also examined by magnetic resonance imaging (MRI: T1-weighted images). A forced duction test was conducted intraoperatively. Observations: Each case presented good visual acuity and neither globe showed any injury. Motility disturbance of the IO was shown in each case by binocular s ingle vision field testing and the Hess chart. The possibility of the incarcerat ion of the IO branch of the oculomotor nerve, which runs from the incarcerated esion to the superior belly of the IO, in an orbital floor trapdoor fracture was shown on CT and MRI. Intraoperative forced duction testing revealed a restricti on due to the incarceration of the connective tissue septa. Conclusions: As infered from the CT andMRI analyzes conducted in this study, IO palsy may be one of the causes of ocular motility disturbance of the IO in an orbital floor trapdoo r fracture, in addition to the ocular motility disturbance due to the connective tissue septa.