Background: Many sedation scales and tools have been developed and compared for validity in critically ill patients. Objective: The aim of this study is to compare the reliability of two sedation scales; RAMSAY sedation scale and Richmond Agitation Sedation Scale (RASS) in the adult intensive care unit. Methods: 290 patients in intensive care unit were recruited for the study and were independently assessed for sedation effect by investigator and bedside nurses simultaneously using RAMSAY scale and RASS scale. Results: Agreement between the nurse and researcher scores on RAMSAY scale (weighted κ=0.449, p<0.001) indicating weak level of agreement. Agreement between the nurse and researcher on RASS scale (weighted κ=0.879, p<0.001) indicating strong level of agreement. Cronbach’s alpha analysis showed that 10 items of RASS had excellent level of internal consistency (α=0.989) compared to good level of internal consistency of RAMSAY scale (α =0.828). Conclusion: RASS showed excellent inter-rater agreement compared to weak inter-rater agreement of RAMSAY scale. The results also support that RASS has consistent agreement with clinical observation and practice among different observers. The results suggest that use of RASS is linked to more reliable assessment of sedation levels in the ICU.