A 45-year-old woman presented with a rare case of small isolated Onodi cell mucocele manifesting as unilateral chronic optic neuropathy. The patient complained of gradual visual disturbance of her left eye with no other neurological abnormalities. Neuroimaging examinations showed a small cystic lesion located in the left posterior ethmoid extending into the anterior clinoid process. Surgery via the pterional epidural approach revealed a cystic lesion containing mucinous fluid isolated in the anterior clinoid process. Total removal of the lesion combined with unroofing of the left optic canal resulted in relief of the patient's symptom. Histological examination showed the lesion was a mucocele. Based on the radiological and surgical findings together with the histological examination, the final diagnosis was Onodi cell mucocele. Such small Onodi cell mucocele can cause optic neuropathy but is extremely unusual.