Background: Pain assessment and documentation can be used as vital sign for the improvement of postoperative pain management; however, these systems have never been adopted for use at Srinagarind Hospital. Objectives: To develop a systematic postoperative pain assessment and documentation as the fifth vital sign; and, To implement a revised graphic sheet for pain documentation at Srinagarind Hospital. Design: Descriptive study Setting: Srinagarind Hospital, Faculty of Medicine, Khon Kaen University Methods: Researchers explored the ward nurses’ attitudes and practices vis-à-vis the pain assessment and documentation in a pilot ward. Subsequently, systematic pain assessment and documentation using the old graphic sheet, originally developed for recording four other vital signs, were re-designed to include the new sign, tested and revised for its practicality. The revised graphic sheet was re-tested in two other wards and finally the system was implemented throughout the hospital. Measurement: Pain assessment tools, frequency of pain documentation and the number of patients assessed before and after the study were recorded. The ward nurses were asked to give their comments on the practicality of the new system for pain assessment and documentation. Results: In the pilot ward, misunderstanding regarding the pain assessment tools was found among some ward nurses. The frequency of pain assessment and documentation significantly increased after using the revised graphic sheet (43.2% vs. 57.3%, p=0.039). The prevalence of no pain assessment also decreased significantly (12.1 vs. 2.2%, p=0.008). Most of the nurses (87.5%) could complete the pain record form within one minute. The new system, as reported by 62.5% of the ward nurses, nominally increased their workload. Re-testing this system in the other two wards increased the number of pain assessments and documentations on the first post-operative day from 67% to 98 %. The revised graphic sheet was then accepted by the policy makers for use throughout the hospital. Conclusion: The system of post-operative pain assessment as the fifth vital sign was developed. A revised graphic sheet was used for pain documentation and accepted for routine use. The new system increased the number of pain assessments and documentations of post-operative patients.