Hypoplasia of the internal carotid artery is a rare congenital anomaly. Agenesis, aplasia, and hypoplasia of the internal carotid artery [ICA] are frequently associated with cerebral aneurysms in the circle of Willis. Authors report two cases with congenital hypoplasia of the ICA accompanying with the aneurysms. Transfemoral cerebral angiography [TFCA] in one patient identified nonvisualization of the left ICA. Bilateral anterior cerebral artery [ACA] and middle cerebral artery [MCA] were supplied from the right ICA accompanying with two aneurysms at anterior communicating artery [AcoA] and A1 portion of the left ACA. TFCA in another patient demonstrated hypoplastic left ICA and left ACA filled from the right ICA accompanying with AcoA aneurysm. Left MCA was filled from basilar artery via posterior communicating artery [PcoA]. Skull base computed tomography [CT] in two patients showed hypoplastic carotid canal. Authors performed direct aneurysmal neck clipping. Follow up CT angiography [CTA] at one year after surgery did not show regrowth or new development of the aneurysm. In patients with hypoplastic ICA, neurosurgeons should be aware of the possibility of development of the aneurysms, presumably because of hemodynamic process. Direct aneurysmal neck clipping is a good treatment modality. After operation, regular CTA, magnetic resonance angiography [MRA] or TFCA is needed to find progressive lesion and to prevent cerebrovascular attack [CVA].