Introduction: A consistent approach to pain assessment for patients admitted to intensive care unit (ICU) is a major difficulty for health practitioners due to some patients' inability, to express their pain verbally. This study aimed to assess pain behaviors (PBs) in traumatic brain injury (TBI) patients at different levels of consciousness. Methods: This study used a repeated-measure, within-subject design with 35 patients admitted to an ICU. The data were collected through observations of nociceptive and non-nociceptive procedures, which were recorded through a 47-item behavior-rating checklist. The analyses were performed by SPSS ver.13 software. Results: The most frequently observed PBs during nociceptive procedures were facial expression levator contractions (65.7%), sudden eye openings (34.3%), frowning (31.4%), lip changes (31.4%), clear movement of extremities (57.1%), neck stiffness (42.9%), sighing (31.4%), and moaning (31.4%). The number of PBs exhibited by participants during nociceptive procedures was significantly higher than those observed before and 15 minutes after the procedures. Also, the number of exhibited PBs in patients during nociceptive procedures was significantly greater than that of exhibited PBs during the non-nociceptive procedure. The results showed a significant difference between different levels of consciousness and also between the numbers of exhibited PBs in participants with different levels of traumatic brain injury severity. Conclusion: The present study showed that most of the behaviors that have been observed during painful stimulation in patients with traumatic brain injury included facial expressions, sudden eye opening, frowning, lip changes, clear movements of extremities, neck stiffness, and sighing or moaning.