Pulmonary arterial hypertension (PAH) is a progressive disease. In recent years, phosphodiesterase type 5 inhibitors such as sildenafil have been used to treat this disease in children. Recently, tadalafil has been used in adults with similar efficacy but it has been used less often in children. This experimental study was carried out in 18 known patients aged 4-24 years in the Emam Hossein Hospital of Isfahan, Iran. All patients had been taking sildenafil for a few months to years. Patients underwent echocardiographic study, the 6-minute walk test (6MWT), and non-invasive pulse oximetry before and after the 6MWT. These tests were repeated again after sildenafil had been switched to tadalafil for 6 weeks. After 6 weeks of tadalafil prescription, the severity of some of the patients' symptoms decreased, but the New York Heart Association class of the patients did not change more. Mean ± standard deviation (SD) oxygen saturation while taking sildenafil and after 6 weeks of tadalafil were significantly different (p = 0.005). Furthermore, mean ± SD oxygen saturation after the 6MWT while taking sildenafil and after 6 weeks of tadalafil were significantly different (p = 0.036). The mean ± SD distances walked in this test while taking sildenafil and tadalafil were significantly different (p = 0.005). No significant side effects were seen; 15 patients continued tadalafil. Tadalafil may be a safe drug to treat children and young adults with PAH. We did not observe any significant side effects during usage; it improves functional capacity and oxygen saturation better than sildenafil in these patients, and requires fewer daily doses than sildenafil.