Chiari malformation (also known as submicrocephalic herniation malformation) combined with hydrocephalus has a treatment sequence problem. Should the hydrocephalus be treated first? Chiari malformation can cause hydrocephalus, which in turn can contribute to the worsening of the submicrocephaly. Which treatment is the least invasive? Which treatment has the potential to address both aspects of the problem? In addition to routine tests, cerebrospinal fluid circulation dynamics should be performed to understand the site of cerebrospinal fluid circulation obstruction and to assess the postoperative outcome. Generally, hydrocephalus is treated first, intraventricular pressure in the fourth ventricle decreases, subcerebellar tonsillar herniation may improve, and the procedure is less invasive. Previously, ventriculo-peritoneal shunts were used to treat hydrocephalus with less trauma, but lifelong tubes were required and complications such as infection, shunt blockage, fracture, and displacement could occur. We use endoscopic third ventriculostomy to treat hydrocephalus, which is effective and does not require shunt placement, and can avoid various complications of shunt surgery.