Background Postoperative pain is significant in cardiac surgical patients. Perioperative analgesia with intermittent administration of opioids can result in significant fluctuations in serum opioid concentrations. Methadone should provide a rapid onset and long-term pain relief upon a single intravenous dose at induction of anaesthesia, and may reduce chronic postsurgical pain (CPSP) in cardiac surgical patients. The feasibility of using intravenous methadone in Chinese cardiac surgical patients, and its effect on acute and chronic pain management after cardiac surgery will be evaluated. Methods A single-centre, prospective, randomized-controlled study. Adult cardiac surgical patients will be randomized to receive either methadone 0.2mg/kg (maximum dose 20mg) or equipotent dose of morphine at induction of anaesthesia. Patient-controlled analgesia morphine protocol, oral paracetamol and dihydrocodeine will be given for postoperative analgesia. Venous blood sampling for plasma methadone concentration will be obtained at regular intervals from study drug infusion to 96 hours after administration. The primary outcomes will be the recruitment and attrition rate, complications from methadone, and the perceived trial feasibility. Secondary outcomes include time weaned to spontaneous breathing trial, morphine consumption within 24 hours and 72 hours after surgery, time to first morphine rescue, postoperative pain score, patient satisfaction, length of stay in ICU and hospital. Side effects of opioids including sedation, nausea and vomiting, and time to first bowel opening will be recorded. CPSP will be assessed with DN4 questionnaire at 3 and 6 months after surgery.