Background: Endotracheal suctioning is a common procedure for intubated patients. In Songklanagarind hospital, the sterile technique for endotracheal suctioning is used routinely in all areas except in the operating theater. Objective: To compare bacterial contamination from endotracheal secretion between the clean and the sterile technique. Methods: This prospective randomized controlled trial in 276 patients of age more than 18-years-old who had American Society of Anesthesiologists (ASA) class I-III and scheduled for elective surgery and had no risk of aspiration were enrolled and randomly assigned into two groups. The patients with general anesthesia with oroendotracheal intubation were included. Endotracheal suctioning was performed by the decision of anesthesiologists or anesthetic nurses with sterile or clean technique regarding the group assignment. Secretion collected from the suction catheter was sent to the microbiology laboratory for microbial culture. The microbiologic bacterial results were reported within 7 days. Patients were followed up for respiratory symptoms every day for 7 days after the surgery. Results: There were the non-pathogenic and pathogenic bacterial growth, and fungus from the culture of endotracheal secretion similarly in both clean and sterile groups. The clean group had positive bacterial growth of 64.2% and the sterile group had 57%. The difference was not statistically significant (p = 0.346). The contamination score in the clean and sterile group were 19 and 15 respectively. The incidence of respiratory symptoms such as a cough without fever was low (5%) postoperatively and there was no postoperative pneumonia in both groups. Conclusion: The incidences of bacterial contamination from endotracheal suctioning with clean and sterile technique were similar. But the clean group tended to has higher contamination than the sterile group.