Arachnoid cysts account for about 1% of all intracranial tumors, in which about 50% arise from the middle cranial fossa, and rarely occur at the cerebral convexity. They sometimes are associated with chronic subdural hematoma (CSDH) but the exact mechanism of their development is still unclear. A 15-year-old boy was admitted to our hospital with an arachnoid cyst at the right frontal convexity. When he experienced recurrent generalized seizure, CT and MRI revealed CSDH localized at the surface of the arachnoid cyst. Radical operation for the cyst with CSDH was successfully performed. As far as we know, it is rare that CSDH and hematoma are colocalized over an arachnoid cyst at the cerebral convexity. In this report, the operative findings of this patient and possible mechanism of CSDH formation were reported.