The study aimed to evaluate mean platelet volume (MPV), platelet distribution width (PDW), and platecrit in children with pulmonary arterial hypertension associated with congenital heart disease (PAH-CHD), to assess the predictive value of these platelet activation markers for adverse outcomes, and to correlate their levels with various data in these patients. This prospective cohort study included 60 children with PAH-CHD as group I and 60 children with CHD and no PAH as group II. Another 60 healthy children of matched age and sex served as the control group. All included children were evaluated by echocardiography. MPV, PDW, and platecrit were also measured using an automated blood counter. All patients were followed up for death or readmission for 6 months. MPV, PDW, and platecrit were significantly higher in group I compared to group II and the control group and they correlated well with increasing severity of PAH. MPV, PDW, and platecrit positively correlated with right ventricular diameter and mean pulmonary artery pressure, however they correlated negatively with right ventricular systolic and diastolic function. The best cut-off of platelet activation markers levels to predict poor prognosis in group I was > 11.2 FL with 75% sensitivity and 96.6% specificity for MPV, > 12.7 FL with 75% sensitivity and 61.5% specificity for PDW, and > 0.505% with 75% sensitivity and 93.2% specificity for platecrit. MPV, PDW, and platecrit were elevated in children with PAH-CHD and found to be good predictive markers for poor prognosis in these children.