Background and Objective: Hypotension associated with spinal anesthesia for cesarean section is still a clinical problem. The role of crystalloid preloading to prevent hypotension associated with spinal anesthesia in parturients during cesarean section has been challenged. However, studies with crystalloids predict that fluid loading should be more efficacious if administered immediately after induction of spinal anesthesia. The effects of colloid loading after spinal anesthesia in cesarean section have not been studied enough. The aim of this study was to compare pre and co-loading of hetastarch for the prevention of hypotension following spinal anesthesia for cesarean delivery. Materials and Methods: This randomized clinical trial study was performed in 112 parturients (ASA I or II) undergoing elective cesarean section. Patients were randomly allocated to one of the two groups to receive rapid infusion of 500 ml of 6% hydroxyethylstarch (HES) before spinal anesthesia (preloading group, n = 56), or rapid infusion of 500 ml of HES after induction of spinal anesthesia (co-loading group, n= 56). The incidence of hypotension and the amount of vasopressor, (ephedrine 5 mg/mL + phenylephrine 25 micg/mL) were compared in the treatment of hypotension. Results: There was no significant difference in hypotension between the two groups (P = 0.58). The pre-loading group used 2.2 ± 1 ml of vasopressor mixture compared with 1.7 ±0.7 ml in the co-loading group (P = 0.04) and the difference was significant. Conclusion: Colloid loading after induction of spinal anesthesia is as effective as preloading in reducing hypotension in cesarean section.