Objective: The causes and factors related to the high concentration of waste anesthetic gases (WAGs) and the risk level of exposure among nurse anesthetist in operating rooms were investigated. Material and Method: A cross-sectional study was conducted in Songklanagarind Hospital, Thailand during June 2010 – July 2012. The WAGs concentrations were monitored in 14 operating rooms and 1 recovery room. A semi-quantitative risk assessment was used to assess the exposure to WAGs. The observation checklists were used to clarify the substandard conditions of working facilities and practices. Results: Nitrous Oxide (N2O) and sevoflurane were commonly used anesthetic gases during the study period, with the 8-h time weight average concentrations ranging from 0-26.69 and 0-6.04 ppm, respectively. The main causes of exposure were due to the unfitted mask to the pediatric patients’ face, the use of the Jackson Rees Circuit, and the leakage from circuit disconnection. The nurse anesthetists in pediatric surgical room exposed the highest levels of both N 2 O and sevoflurane which were 127.5 and 125.5 ppm, respectively, even the scavenging system were functioning. Conclusion: This study illustrates that WAGs levels in operating room depended on the anesthesia practices. The risk assessment of WAGs exposures of the nurse anesthetists were at the low to medium- risk level. The standardized work instructions, appropriated anesthesia circuit, air circulation and ventilation system should be improved.