Eighty patients with osteoarthritis who underwent unilateral total knee arthroplasty were randomly assigned to two groups: the trial group received an intra-operative intra-articular injection of magnesium sulphate and ropivacaine, and the control group received an injection of normal saline. All patients received patient-controlled analgesia with morphine for 48 h post-operatively. It was found that an intra-articular injection of magnesium sulphate and ropivacaine significantly reduced morphine consumption during the 0 - 24 h post-operative period and total 48-h post-operative morphine consumption. Pain scores at rest and during motion in the trial group were significantly lower than in the controls during the first 24 h post-operatively. The time to be able to perform a straight leg raise and to reach a 90 degrees knee flexion was significantly shorter in the trial group compared with the controls. This study demonstrated that an intra-operative intra-articular magnesium sulphate and ropivacaine injection reduced the use of post-operative morphine.