Abstract: Objective 　To compare rectal administ ration with oral administ ration in the t reatment of cancer pain and the side effect s. Methods 　One hundred and twenty patient s with cancer pain were randomly di2 vided into 4 group s with 30 patient s in each group. The patient s were given sustained2release morphine tablet s with total dosage of 30～60 mg/ d, twice daily. OR group was administered orally in the first two weeks and rectally in the later two weeks ; RO group was administered rectally in the first and second weeks, orally in the third and th fort weeks ; OO group was administered orally in four weeks ; RR group was administered rectally in four weeks. The analgesic efficacy and the side effect s of nausea, vomiting and constipation were compared. Results 　Af ter the patient s given sustained2release morphine tablet s, RR was 77 % (OR group), 80 %(RO group), 82 % (OO group), 77 % (RR group) . The occuring rate of nausea was 24 % (OR group), 3 %(RO group), 33 % (OO group), 18 % (RR group) ; vomiting was 16 % (OR group), 13 %(RO group), 20 % (OO group), 3. 3 % ( RR group ), constipation was 20 % (OR group), 14 %(RO group), 33 % (OO group), 20 % (RR group) . Conclusion 　The patient s given sustained2release morphine tablet s with rectal administ ration got the same analgesic effect s and less adverse reactions com2 pared with the patient s given sustained2release morphine tablet s with oral administ ration.