Abstract: Objectives　To evaluate the application of gastric intramucosal pH values in assessment of the gastric mucosal lesions in infants and toddlers with severe pneumonia. Methods　One hundred and twenty (120) infants and toddlers with severe pneumonia were recruited. All subjects were treated with pressure support - synchronized intermittent mandatory ventilation according to the condition. The blood gas analysis and gastric carbon dioxide partial pressure (PgCO2) detection were performed at 15 minutes before ventilation and l h, 6 h, 12 h, 18 h, 24 h after ventilation, and the gastric intramucosal pH were calculated. Results　With prolonged ventilation, the gastric intramucosal pH, PaO2 and PaO2/fraction of inspired oxygen (FiO2) were significantly increased, while the (PgCO2-PaCO2) and blood lactic acid were significantly decreased. The (PgCO2-PaCO2), PaO2, PaO2/FiO2, and blood lactic acid began to return normal at 18 h after ventilation, and close to normal at 24 h after ventilation. The gastric intramucosal pH was still significantly lower than normal at 24 h after ventilation, and return to normal at 36 h after ventilation. There were negative correlations between gastric intramucosal pH with (PgCO2-PaCO2) and blood lactic acid (r=-0.30, -0.23, P<0.05), and positive correlations between gastric intramucosal pH with PaO2 and PaO2/FiO2 (r=0.26, 0.16, P<0.05). Conclusion　Gastric intramucosal pH can be served as a sensitive and accurate indicator for gastric mucosal lesions of severe pneumonia in infants and toddlers who need mechanical ventilation.