Purpose: The purpose of this study was to examine the effect of three kinds of modes using bolus button of PCA on level of pain and side effects of analgesic and amount of drug consumption in post-operative patients according to whether the medication is controlled by the patient, the caregiver or the nurse. Method: The participants were 684 patients using PCA after an operation. The data collection period was from March 19 to April 6, 2007. Results: It was found that there were statistical differences in gender, age, type of surgery, pain on first post-operative day, amount of drug consumption, nausea, and vomiting. The ratio for patient controlled medication was 55.7% for women, and 70.5% for men, and for care-giver controlled medication, 35.1% for women, and 20.0% for men. Average pain scores for the first post-operative day were $3.9{pm}2.2$ for patient controlled medication and $4.5{pm}2.3$ for care-giver controlled medication. There were statistical differences according to mode used for PCA for amount of drug consumptions, nausea and vomiting but not for pain, operation day or pruritus. Conclusion: This study was carried out to examine risks according to who controls the PCA for post-operative patients. The results can help to develop education program for everyone who is involved in PCA, patients, caregivers, nurses and doctors.