Purpose: This study was to investigate the effect of the application of cancer pain management guidelines on pain management among patients in Cancer Emergency Room. Methods: This study was a retrospective descriptive study. Before application, data were collected by analyzing the Electronic Medical Record in Cancer Emergency Room in September, 2011, and after application in February, 2012. The subjects of this study consisted of 231 patients (pre-application group 83, post-application group 148), who stayed over 24 hours and complained of pain higher than Numeric Rating Scale score 4. The post-test was conducted after educating the nurses about the application of the pain management guidelines in the Electronic Medical Record. Results: This survey showed that, as the cancer pain management guidelines were applied for cancer patients with above moderate pain, the pain intensity decreased, the number of patients reaching the treatment goal score increased. Furthermore, the estimated time to reach the treatment goal decreased significantly. Conclusion: Pain intensity of the cancer patients was decreased through regular pain assessments by nurses and the medication of analgesics according to the cancer pain management guidelines. Therefore, it is necessary to develop the pain management program and to provide the physicians and nurses with intensive education about the pain management guidelines for systematic and effective pain management.