To determine the efficacy and safety of oxycodone i.v. patient-controlled analgesia (PCA) in the pain relieving treatment during 48h postoperative period, by comparing with morphine i.v. PCA.

Inclusion Criteria:

1. Patients of either sex aged 18 to 65 years old, with a standard body weight [standard body weight = height (cm) - 100] ±15%.
2. ASA I and II.
3. Scheduled on elective open upper abdominal surgery under general anesthesia (liver, gall bladder, spleen, stomach, kidney, colon surgery), of which predicted operation duration is between 2-4h.
4. Hospitalized patients on non-emergency condition who have been given appropriate preoperative treatments.
5. Patients who are willing to participate in the study and have signed the written informed consent.
6. Negative pregnancy test result should be obtained for women of child-bearing age.

Exclusion Criteria:

1. Long-term analgesics or psychotropic drugs (including opioids, NSAIDs, sedatives, antidepressants) taken for the chronic pain, or patients abusing alcohol.
2. Body weight is less than or over ±15% of the standard body weight.
3. Severe impairment of liver and renal function at preoperative stage (ALT, AST, BUN, Cr).
4. Medical history of recovering from abnormal surgery anesthesia.
5. Medical history of hypertension (Systolic blood pressure ≥180Hg, Diastolic blood pressure 110Hg).
6. Esophagus reflux disease.
7. Sedatives, anti-emetics, and anti-pruritic drugs were taken 24 hours before operation.
8. Have known hypersensitivity to opioids.
9. Monoamine oxidase inhibitors and antidepressant drugs were taken within 15 days.
10. Patients with shock.
11. Patients with COPD.
12. Patients can not understand the VAS or unable to use PCA.
13. Pregnant or parturient women.