Pulmonary arterial hypertension (PAH) is a rare disease but with significant morbidity and high mortality. There is no specific way to diagnose PAH. Thus, an easy used with good sensitivity and specificity biomarker of PAH is highly desirable to aid in the screening, diagnosis, and follow‐up. Caveolin‐1 (Cav1) is the structural protein of caveolae and is highly expressed in type I pneumocytes. Lungs tissues from idiopathic PAH (IPAH) patients showed decreased expression of Cav1 in vascular endothelial cells. Therefore, we developed a direct sandwich immunoassay for the determination of Cav1 in IAPH patient’s serum. The result disclosed serum Cav1 level was significantly lower in IPAH than control groups. Using serum Cav1, 17.17 pg/mL as a cutoff value, the sensitivity was 0.59 and the specificity was 1.0. There were two major findings in our results. First, serum Cav1 might be a novel biomarker in the diagnosis of IPAH with fare sensitivity and good specificity. Second, Cav1 might be used to make differential diagnosis between COPD‐PH and IPAH group.