A series of novel pyrazolo[4,3-d]pyrimidin-7-ones was designed and synthesized in order to find new potent phosphodiesterase 5 inhibitors(PDE5). The structures of all the compounds were confirmed by IR, 1H NMR and MS; their in vitro vasodilatory activities were tested on isolated rabbit aortic spiral strips. Among them, compound Ⅹ3 displays a significant vasodilatory activity, which is more potent than verapamil.