Pain assessment in critically ill patients, particularly those who are mechanically ventilated and unable to self-report, remains a significant challenge in intensive care practice. Effective pain management is central to improving patient comfort, reducing physiological stress, minimizing complications, and enhancing recovery. However, conventional self-reporting methods are not feasible in sedated or intubated patients, making reliance on observational and behavioral assessment tools essential. Several behavioral pain assessment scales have been developed and validated internationally, including the Critical-Care Pain Observation Tool (CPOT), Behavioral Pain Scale (BPS), and Non-Verbal Pain Scale (NVPS). Each of these tools measures pain through a combination of behavioral indicators and, in some cases, physiological parameters. Although they have been used in diverse ICU populations, their comparative performance in post-surgical mechanically ventilated patients within the Indian context has not been studied to the best of our knowledge. This prospective observational study will compare three behavioral pain assessment tools CPOT, BPS and NVPS in 106 mechanically ventilated post-surgical ICU patients. Pain assessments will be conducted at baseline and during standard nociceptive ICU procedures, including repositioning, endotracheal suctioning, and wound/drain management. Physiological indicators, specifically a greater or equal to 20 percent increase in heart rate or mean arterial pressure from baseline, will be used as a gold standard to evaluate the diagnostic accuracy of each tool. The primary objective is to compare the reliability, sensitivity, and specificity of CPOT, BPS and NVPS against physiological responses. The secondary objective is to analyze the internal consistency and component-level responsiveness of each scale to determine the most predictive behavioral indicators of pain. By addressing this gap, the study seeks to identify the most reliable and valid behavioral pain assessment tool for use in post-surgical ICU patients. The findings are expected to contribute to standardized pain assessment protocols, improve clinical decision-making and enhance the overall quality of critical care delivery in the Indian healthcare setting.