Objective To evaluate the prediction and evaluation of plasma D-dimer and fibrinogen levels upon the severity of community acquired pneumonia(CAP). Methods Clinical variables of 180 patients with CAP were evaluated and divided into different groups by CURB65.Plasma D-dimer and fibrinogen were measured to compare their levels among different groups and evaluate the prediction in the diagnosis of the severe CAP by ROC curve. Results The mean D-dimer and fibrinogen level were different significantly among different CURB65 groups. D-dimer and fibrinogen level of severe CAP were significantly higher than non-serious pnrumonia group. The area under the curve of the D-dimer, fibrinogen in the diagnosis of severe pneumonia were 0.815, 0.777. Conclusion Plasma D-dimer and fibrinogen can effectively evaluate the severity of illness in patients with community acquired pneumonia and they could be useful for assessment of the severity of CAP.