Previous studies suggest that combinations of existing therapies may be effective for pulmonary arterial hypertension (PAH). However, all of these studies are sequential combination therapy, for example, by adding sildenafil to previously prescribed bosentan. This kind of therapy model is not enough for PAH patients, especially those with New York Heart Association (NYHA) class Ⅲ and Ⅳ. In this randomized, multicenter study, the investigators evaluate the safety and efficacy of combining inhaled iloprost, a prostacyclin analog, with the endothelin receptor antagonist bosentan in treatment naive patients with PAH by comparing with bosentan monotherapy. Efficacy endpoints include change from baseline in 6-min-walk distance (6-MWD), modified (NYHA) functional class, hemodynamic parameters, and time to clinical worsening.