Transcranial doppler is a screening test to detect patients with sickle-cell disease who has high cerebral blood flow velocities. Prophylactic blood transfusion and hydroxyurea recommended for patients with cerebral blood velocities higher than 200cm/m to prevent stroke. It was necessary to have more research to find the effect of hyroxyurea on cerebral blood velocity in these descriptive research 42 patients with sickle-cell disease from 2-14years old were enrolled as cases and 20 healthy children without hematologic disease as controls. Transcranial doppler has been done for both group. Cerebral blood velocity in Medial Cerebral Artery was more in cases in compare to controls. Systolic blood velocity in all right and left cerebral arteries was more in cases in compare to controls. Blood velocity in right and left artery in 78.4% in cases was more in compare to 8% in controls; It shows the significant difference between cases and control. Higher blood velocity in cases was same as other researches but there was no difference between velocity of blood in right and left artery in compare to other researches. In caseses who has been treated with hydroxyurea blood velocity was less in compare to those who didn't take it which was same as other researches.