Objective To analysis the risk factors for lower airway bacteria colonization and ventilator-associated pneumonia ( VAP) in mechanically ventilated patients. Methods A prospective observational cohort study was conducted in intensive care unit. 78 adult inpatients who underwent mechanical ventilation( MV) through oral endotracheal intubation between June 2007 and May 2010 were recruited. Samples were obtained from tracheobronchial tree immediately after admission to ICU and endotracheal intubation( ETI) , and afterward twice weekly. The patients were divided naturally into three groups according to airway bacterial colonization. Their baseline characteristics, APACHEⅡ score, intubation status and therapeutic interventions, etc. were recorded and analyzed. Results In the total 78 ventilated patients, the incidence of lower airway colonization and VAP was 83. 3% and 23. 1% , respectively. The plasma albumin( ALB) ≤29. 6 g/L( P lt; 0. 05) , intubation attempts gt; 1( P lt; 0. 01) were risk factors for lower airway colonization. In the patients with lower airway colonization, preventive antibiotic treatment, applying glucocorticoid and prealbumin( PA) ≤ 69. 7 mg/L were risk factors for VAP ( P lt; 0. 05) . Conclusions The risk factors for lower airway colonization in ventilated patients were ALB≤29. 6 g/L and intubation attempts gt; 1. And for lower airway colonized patients, PA ≤ 69. 7 mg/L, preventive antibiotic treatment and applying glucocorticoid were risk factors for VAP.