Purpose: The purpose of this study was to examine the reliability and validity of a Korean version of the Critical Care Pain Observation Tool (CPOT) developed for assessment of pain in critically ill nonverbal patients. Methods: Data were collected from a convenience sample of 30 critically ill patients admitted to a medical ICU in a hospital. The CPOT was tested at before, during and 20 minutes after changing a position and suction. Upon establishment of content and translation equivalence between the English and Korean version of CPOT. Results: The interrater reliability was found to be acceptable with the kappa coefficients of .76-1. The construct validity of the pain scores were increased from 0.43 to 2.5 in changing a position (t=-8.60, p<.001)and 0.1 to 3.23 (t=-9.36, p<.001) in suctioning. The pain scores were decreased from 6.06 to 4.01 in changing a position (t=-10.19, p<.001) and 6.45 to 4.13 (t=-10.39, p<.001) in suctioning. The concurrent validity the correlations between pain scores and physiological indicators, and a increased in Heart rate before and after changing a position (r=.65, p<.001). Conclusion: The CPOT can be used as a reliable and valid measure to assess pain in critically ill nonverbal a medical ICU patients.