heart failure, left to right shunt congenital heart defect, carvedilol, cardio-thoracic ratio.Background: Heart failure due to left to right shunt congenital heart defect will increase pulmonary blood flow and cause volume overload. This condition activated the sympathetic nervous system and the reninangiotensin-aldosteron system. Conventional therapy has not blocked the sympathetic system yet. Carvedilol, a novel non selective β-blocker, reduced mortality and hospitalization in adults with heart failure. Limited information was available about its use in children. Objective: To evaluate the effect of carvedilol on chest x-ray in children with heart failure due to left to right shunt congenital heart defect. Methods: A randomized, double-blind, placebo-controlled study was done. In addition to conventional therapy, patients were assigned to receive placebo or carvedilol. Carvedilol was initiated at a dose of 0.05 mg/kg/day, with a target dose of 0.2 mg/kg/day. Chest x-ray was done before-after treatment and evaluated for cardio-thoracic ratio and pulmonary vascularity. The data was analyzed using independent sample t-test and Chi-square test, with confidence interval 95%. Results: Of 30 patients, 15 in each group. The mean age was 57.6(SD 43.57) months, 19(63.3%) were boys. There were 21(70%) children with VSD and 9(30%) children with PDA. Compared to control group, children in the carvedilol group had a significant decrease of cardiothoracic ratio (-2.94±2.34% versus -0.48±3.19%, p=0.023, CI 95%:-4.556 to-0.360). However, there was no significant change of pulmonary vascularity(p=0.153). Conclusion: Carvedilol decreased the cardio-thoracic ratio on chest xray, but did not improve the pulmonary vascularity in children with heart failure due to left to right shunt Congenital heart defect.