Interpretation of pain messages from patients is an important communicative action in the intensive care unit (ICU). This study explored how "pain" is recognized in pain assessment through (a) clinical knowledge, (b) neurocognitive perception, and (c) communicative actions among ICU staff nurses. A 2-phase explanatory sequential mixed-method design was applied. Data are collected from May 14 to 22, 2017 in different government ICUs. Forty female expatriate nurses mostly with baccalaureate degree (82.5%), mean age of 33 years, and mean work experience of 6 years have participated. Five themes were isolated: pain is physical, emotional, or mixed; pain assessment is facial and behavioral/physiological; barriers to pain assessment are related to healthcare team and system; pain assessment functions between task and diagnostic; and pain assessment is valued as task and diagnostic. Pain assessment is usually done at the beginning of the shift (75%) or as needed (25%). Emotional intelligence scores were at average and high levels. Nurses scored pain more often (51.04%) than no pain (48.96%) and had more neutral facial expression (0.6498 msec) when deciphering pain. The communicative meaning of pain assessment is "knowing patient's feeling". Neurocognitive perception of nurses to pain in nonvocal patients is connected to their clinical knowledge and learned practices within the ICU. Clinical training on facial expressions of pain in nonvocal patients should be included.