ObjectiveTo investigate the changes in serum procalcitonin (PCT) levels in patients with different bacterial infections and to evaluate the significance of PCT levels for differentiating grampositive and gramnegative bacterial infection. MethodsTwo hundren and sixtysix intensive care unit（ICU）patients infected with severe bacterial infections were divided into 2 groups (grampositive bacteria group and gramnegative bacteria group) according to the bacterial culture results, and serum PCT, Creaction protein(CRP), white blood cell(WBC) and neutrophil ratio（Neu%） were detected, and data of two groups were analysed statiscally. ResultsSerum levels of PCT in patients with severe bacterial infection significantly rose. When defined by the value of PCT ≥2 ng/mL as positive, the positive rate of PCT in gramnegative bacteria group (92.36%) was significantly higher than that of grampositive bacteria group (76.23%)(χ2=13.45, P<0.001); When the value of PCT ≥10 ng/mL, the possibility of gramnegative bacterial infection was great. The routine infection markers, including CRP,WBC and Neu% , were not significantly different between grampositive bacteria group and gramnegative bacteria group( both P＞0.05）. ConclusionPCT levels can be used as indicator for differentiating grampositive and gramnegative bacterial infection, and may provide valuable information for sensitive antimicrobial selection.