Purpose: The purpose of this methodological study was to examine the reliability and validity of a translated Korean version of the Critical Care Pain Observation Tool (CPOT) developed for assessment of pain in critically ill nonverbal patients. Methods: A cross-sectional study design was used. Data were collected from a convenience sample of 202 critically ill patients admitted to a university hospital. Upon establishment of content and translation equivalence between the English and Korean version of CPOT, psychometric properties were evaluated. Results: The interrater reliability was found to be acceptable with the weighted kappa coefficients of .81-.88. Significant high correlations between the CPOT and the Checklist of Nonverbal Pain Indicators were found indicating good concurrent validity (r=.72-.83, p<.001). Data showed the area under the ROC curve of 0.86 with a cut-off point of 1, which resulted in 76.9% sensitivity and 88.6% specificity. The mean score of CPOT during suctioning was significantly different from that of before (t= -14.16, p<.001) or 20 minutes after suctioning (t=16.31, p<.001). Conclusion: Results of this study suggest that the CPOT can be used as a reliable and valid measure to assess pain in critically ill nonverbal patients.