The purpose of the study was to compare the effects of an educational programon pain assessment knowledge and skill. Seventy-nine surgical nurses were randomly assigned into two groups. The experimental group received an educational program, and there was no education for the control group. Ninety-seven patients cared for by experimental group were also recruited. The results were as follows: The mean age of the experimental group was 28.9±5.4, and 28.7±5.2 for the control group. The mean working year of the experimental group was 69.0±52.7 months, and 61.2±49.0 months for the control group. There were no significant differences between the two groups in demographic variables. The pain assessment knowledge for the experimental group was increased after education (F=17.95, p= .000), the scores were 57.63, 60.86, and 63.12 separately. In pretest, the nurses of the experimental group underestimated the patient's worst pain level (6.19 vs. 7.36, p=.031) and the slightest pain level (2.56 vs. 3.48, p=.042). In post test the nurses overestimated the current pain level (4.00 vs. 2.77, p=.048). In post-post test, the nurses overestimated the light pain level (3.12 vs. 2.09, p=.025), it showed a significant difference. According to the findings, pain assessment in-service education programs can be held regularly to enhance nurse's pain assessment ability.