We report a rare autopsy case of sudden death due to primary pulmonary hypertension. A seven-year-old boy, who had been diagnosed with primary pulmonary hypertension at the age of four years, died suddenly. Forensic autopsy and histopathologic examination revealed extensive obstruction of small muscular pulmonary arteries by plexiform lesions and concentric intimal thickenings, compatible with primary pulmonary hypertension. We concluded that plexiform lesions of pulmonary arteries produced right ventricular hypertrophy and dilatation, decreased the preload of the left ventricle and subsequently led to biventricular failure. This autopsy and histopathologic examination suggested a possible pathophysiologic mechanism of sudden death due to primary pulmonary hypertension in a child.