Objective To investigate the clinical value of the detection of serum procalcitonin (PCT) and plasma D-dimer (D-D) levels in the early diagnosis of infection in critically ill children.Methods Solid-phase immune chromatography and immune nephelometry were used to determine the levels of serum PCT,plasma D-D,plasma C reactive protein (CRP) and WBC count in 302 critically ill children.Results The positive rate of serum PCT and the level of plasma D-D in the infected group (208 children) were significantly higher than those in the non-infected group (94 children)[96.6％(201/208) vs.10.6％(10/94),(1.65 ± 0.73) mg/L vs.(0.45 ± 0.21) mg/L,P ＜ 0.05].But there was no significant difference of plasma CRP level and WBC count between the 2 groups (P ＞0.05).The levels of serum PCT and plasma D-D were positively correlated to the severity of the illness (P ＜ 0.05).Conclusions The levels of serum PCT and plasma D-D are between important sensitivity markers for early diagnosis of critically ill children,illness severity assessment and therapeutic evaluation.The detection of plasma D-D plays an important role in early diagnosis of the degree and the width of coagulation disorder,and also provides the theoretic basis to use anticoagulation drugs for treating the critically ill children with infection.