Abstract: Objective To explore the relationships between antithrombin Ⅲ (AT-Ⅲ) and D-dimer (DD) level with pediatric critical illness score (PCIS) in children with sepsis, and to evaluate the values in assessing the severity of illness. Methods Sixty-one children with sepsis were included in this study. Within 24 hours after admission, blood samples were tested for the activity of AT-Ⅲ and DD level. The PCIS was calculated. According to PCIS, the patients were divided into extremely critical group (<70), critical group (71-80) and non-critical group (80-100). According to the prognosis, the patients were divided into survival and death groups. The differences of the activity of AT-Ⅲ and DD were compared and the relationship with PCIS were analyzed. Results The activity of AT-Ⅲ was lower and DD level was higher in critical group than in non-critical group (P<0.01) and the changes in extremely critical group were more evident than those in critical group. The activity of AT-Ⅲ and PCIS were positively correlated (r=0.548, P<0.01).The DD level and PCIS was negatively correlated (r=-0.657, P<0.01). Compared with survival group, the level of DD was significantly higher in death group (P<0.01), and PCIS and the activity of AT-Ⅲ were significantly lower in death group (P<0.05). Conclusions The patients with sepsis have dysfunctions of coagulation. The activity of AT-Ⅲ, DD level are correlated with illness severity, and can be useful for assessing the severity of sepsis.