The incidence of reported acute pain is still high which indicates a lack of knowledge in clinical pain assessment and management. This study was carried out to investigate strategies of post-operative pain assessment in terms of patterns of criteria adopted and how these are influenced by prior experience. The research approach, phenomenography, was adopted in data analysis. The subjects of this study consisted of 10 nurses from post-operative care units from a University Hospital in Seoul, Korea. Findings revealed that the nurses mostly relied on "how it usually is" and "how the patient looks" as strategies of post operative pain assessment and "I have learned the typology of patients" as a frame of reference of post operative assessment.