The present study aims to study the effects of prophylactic infusion of phenylephrine on mothers’ hemodynamic at Caesarean section with spinal anesthesia. It is a single blind clinical study. 80 pregnant women attending Imam Reza hospital and Motazedi hospital requesting for elective ceasarean section were randomly selected and divided into experiment and control groups, each including 40 students. Data scope passport device , made in America, was used at the beginning of the study to monitor and record vital signs (such as systolic and diastolic blood pressure) non-invasive method, heart bit and oxygen saturation. In the experiment group, 12.5 milligram anesthetic bupivacaine hyperbaric 0.5 was injected intrathecally using 25-spinal needle in L4-L-5 environment and patients immediately were set in supine position. In the control group, Phenylephrine was not injected; in the case of hypotension, common treatment with ephedrine is used to control blood pressure. Immediately after intrathecal injection of the anesthetic, both experiment and control groups were measured and recorded for their vital signs every minute until their delivery, and, also, every three minutes to the end of the surgery by monitoring device. Inclusion: single pregnancy, BMI larger than 20and less than 35, over 36 weeks of pregnancy, patients at their 18 to 40, patients’ satisfaction. Exclusion: coagulopathy or infection in spinal or any other contraindications to spine anesthesia, sensitivity to phynylephrine and bupivacaine, mothers’ hypertension, cardiovascular diseases, cerebrovascular disease, congenital abnormalities of the fetus, diabetic mothers