Little theory-based research has been performed to better understand nurses' perceptions of pain management. Framed by the theory of planned behavior, the aims of the study were to describe nurses' beliefs (behavioral, normative, and control) about pain management for hospitalized elderly patients with postoperative pain; to present an item analysis for beliefs, attitudes, perceived norms, perceived behavioral control, intentions, and behaviors (measured in case study vignettes) for nurses (a) with different durations of nursing experience, (b) working in university, public health, and military hospitals, and (c) who either had or had not received pain management training in the past six months; and to compare differences in the constructs across these three groups. A comparative descriptive cross-sectional design was used with a convenience sample of 140 Thai nurses working in three Bangkok hospitals. Participants responded to pain assessment and management questionnaires. Most nurses expressed fairly strong beliefs about pain assessment and pro re nata (PRN) opioid analgesic administration. Nurses with more than 10 years of experience had the highest scores for attitudes toward pain assessment and perceptions of others’ expectations about PRN opioid analgesic administration. Responses of nurses working in different types of hospitals indicated significantly different pain assessment and PRN opioid analgesic administration behaviors. No significant differences were found for nurses who did and did not receive pain management training. The study highlighted the need for improved pain management education for nurses to enhance the quality of patient care.