Occlusion of cerebrospinal fluid path increases the pressure exerted by the liquid on the walls of the ventricles and ultimately leads to hydrocephalus. This research investigated a numerical index to diagnosis the non-communicating hydrocephalus disease. At first, the diagram of velocity in Sylvius aqueduct of a healthy subject, which was obtained through a 3D FSI analysis, was compared to the similar velocity diagram extracted from CINE-PC-MRI of the same subject. Then after ensuring that the two diagrams coincide with each other, was to make sure that the problem assumptions and solution are correct. The Reynolds number in Sylvius aqueduct of a healthy subject was less than 275.7 and the maximum pressure of CSF was 616.3 Pa. Further, the conditions of ventricular system in a patient suffering from non-communicating hydrocephalus were modeled. The maximum pressure has increased to 2958.5 Pa. Regarding the cause of hydrocephalus, the maximum pressure of CSF on the brain tissue in Sylvius aqueduct was introduced as an index to assess non-communicating hydrocephalus. Finally calculated CSF pressure data of this study were compared to the data obtained through the lumber puncture (LP) test and it was found that these values are proportional to each other. Based on this finding, the CSF pressure obtained by LP test was introduced as a practical numerical index for diagnosis of non-communicating hydrocephalus.