Abstract Oral hygiene is an essential aspect of daily nursing care activity, Oral hygiene in critical often neglected or performed inadequately. Literature cited many oral hygiene solution as Chlorhexidine, Sodium Bicarbonatele, hydrogen peroxide and normal saline. Oral hygiene solutions leads to decrease colonization of the oropharynx, remove debris, and lowers VAP level. Literatures did not mention the most effective method including the solutions and the appropriate frequency of oral hygiene for critical ill patients. The aim of study: is to evaluate the effect of two standardized oral Hygiene methods on bacterial colonization in mechanically ventilated patients. Patients and method: A quasi experimental research was conducted at ICU of Minia university hospital. Sixty adult critical patients assigned randomly in two groups, 30 for each. Chlorhexidine (0.2%) group (A) and Sodium Bicarbonate (0.1%) group (B). Three tools used : Bacterial Colonization Indicators, Beck Oral Assessment Scale & MucosalPlaque scale. Results: on 7th day of the study a higher percentage (87 %) of Chlorhexidine group (A) had good oral integrity, and only (26.7 %) of the same group had VAP than group (B) (73.3 %). Conclusions: Oral hygiene using tooth brushing with Chlorhexidine leads to improve oral cavity reducing colonization as well as VAP prevention.