Objective: To investigate the value of plasma D-dimer as a prognostic marker in severe traumatic brain injury in children and to compare the results of plasma D- dimer level in plasma and the clinical condition of the case and the results of its routine laboratory investigations. Methods: The study was carried out on 66 head trauma children 46 cases and 20 controls of both sexes, who admitted to pediatric intensive care unit in Alharam hospital in Giza. All children patients had full history, vital sign, general examination, complete neurological examination, GCS or PGCS, cranial CT scan, abdominal ultrasonography, full radiological studies, plasma D-dimer on admission, the 3rd day and at 14th day, INR, PT, APTT. on admission, the 3rd day and at 14th day, routine laboratory investigations on admission as CBC, CRP, Liver function test, Urea, Creatinine, Blood glucose, Na, K and Arterial blood gas analysis. Results: GCS improved significantly in the favorable group but not in the unfavorable group. D-dimer mean values were significantly higher in the unfavorable group more than the favorable group. D-dimer is correlated with mortality and can predict poor patient outcome. D-dimer have inverse relation with GCS and. PT, aPTT, INR, and Concentration mean values were significantly higher in the unfavorable group than in the favorable group. Conclusion: GCS or PGCS has evident role in evaluation and assessment of TBI especially in acute stage and gives excellent idea about the prognoses of TBI. D-dimer is a good marker to predict outcome in TBI & it has an inverse correlation with GCS or PGCS. PT, aPTT, INR and Concentration have some role in TBI and their values increase in the acute stage of TBI.