Background & Aim: Although trachea-bronchial suctioning (TBS) is one of the important nursing procedures in intensive care units (ICU), it may be associated with some complications. Using closed system suctioning (CSS) is one of the ways to decrease the rate of complications due to continued ventilation and oxygenation at the time of suctioning. However, CSS' secretion removal is not efficient enough. Higher values of suction pressure have been recommended to enhance the efficacy of CSS. The aim of this study was to compare the efficacy of two levels of negative suction pressure in secretion removal of CSS used for mechanically ventilated (MV) patients. Materials & Methods: Fifty eligible adult MV patients (twenty in each group) with Random allocation participated in this clinical trial study with cross-over design. Each patient was suctioned using CSS, connected to a central suction device, with 100 and 200 mmHg pressures with a two-hour interval. Efficacy of suctioning was measured by the absence of secretion flow at the end of suctioning. Volume of the secretions was measured and compared in each suctioning. Statistical analyses were done using Minitab and SPSS software considering the significance level of 0. 05. Results: CSS using 200 mmHg resulted in an efficacy of 96% for removing secretions, compared to 34% for 100 mmHg (P = <0. 0001). Suctioning volume was increased significantly higher with 200 mmHg suction pressure compared to values with 100 mmHg (1. 72 [95% confidence interval (CI): 1. 4; 2. 0]; P<0. 0001). Conclusion: Application of CSS with Suctioning pressure 200 mmHg is recommended for trachea-bronchial suctioning in mechanically ventilated patients, because nearly complete removal of respiratory suctioning in most subjects.