Objective: using plain bupivacaine with dexamethasone in spinal anesthesia is a new technique in total hip replacement operation. It provide good sensory and motor block with less complication. Dexamethasone is a potent corticosteroid drug was mixed with bupivacaine. This study was done to compare the incidence of side effects of spinal anesthesia, the duration of both sensory and motor block following administration of heavy bupivacaine alone with administration of plain bupivacaine mixed with dexamethasone. Materials and Methods: 100 patients whose ages between 40 &90 years and weight between 70 &110 KG and American society of anesthesia (ASA) 2/3/4 who underwent total hip replacement under spinal anesthesia were randomly allocated into 2 groups by sealed envelope method, each group with 50 patients. First group received 15 mg heavy spinal bupivacaine alone while the second group received 15 mg heavy bupivacaine mixed with dexamethasone. Results: The duration of both sensory and motor block was longer in dexamethasone group in comparison to control group (p<0.001). The incidence of hypotension, breadycardia and shivering was less in dexamethasone group. Conclusion: Dexamethasone when mixed with plain bupivacaine prolong the duration of sensory and motor block and reduce the incidence of hypotension, Breadycardia and shivering.