Phosphodiesterase 5 (PDE5) inhibitors can be used for the treatment of erectile dysfunction and pulmonary hypertension. In order to search for new leads of PDE5 inhibitors, we investigated the chemical constituents of the tubers of Eulophia macrobulbon (E.C. Parish & Rchb. f.) Hook. f. A new phenanthrene, 9,10-dihydro-4-(4'-hydroxybenzyl)-2,5-dimethoxyphenanthrene-1,7-dio (1) and three known phenanthrenes i.e., 1-(4'-hydroxybenzyl)-4,8- dimethoxyphenanthrene-2,7-diol (2), (9,10-dihydro-2,5-dimethoxyphenanthrene-1,7-diol (3) and 1,5,7-trimethoxyphenanthrene-2,6-diol). (4) were isolated Among these, 2 was the most potent PDE5 inhibitor (IC₅₀ =1.67±0.54 μM) evaluated by the [3H]cGMP radioassay method, whereas 1 showed mild activity (IC₅₀ = 62.3±3.3 μM). Their inhibitory selectivities against PDE5 over PDE6 were also studied. This study suggests phenanthrenes as a new class of PDE5 inhibitors.