Flow diverters (FDs) have been developed for intracranial aneurysms difficult to treat with conventional endovascular therapy and surgical clipping. We reviewed 94 patients with 100 large or giant unruptured internal carotid artery (ICA) aneurysms treated with Pipeline embolization device (PED) embolization from December 2012 to June 2017 at Juntendo University Hospital. The patients’ mean age was 63.4 years (range, 19–88), and there were 90 women 89.4%. Aneurysm locations were: C4 (45), C3 (4), and C2 (51) in ICA segments. Mean aneurysm size and neck width were 16.9 ± 6.8 mm and 8.3 ± 4.4 mm, respectively, in 40 symptomatic and 60 asymptomatic aneurysms. Follow-up catheter angiographies of 85 patients with 90 aneurysms showed no filling in 62 aneurysms (68.9%), entry remnant in 16 (17.8%), subtotal filling in 11 (12.2), and total filling in 1 (1.1%) with a mean follow-up of 10.2 ± 5.6 months. In-stent stenosis occurred in 1 patient and parent artery occlusion in 2 during follow-up. Hemorrhagic complications occurred in 4 (4.3%): delayed aneurysm rupture (2) and intraparenchymal hemorrhage (2). Ischemic complications with neurological symptoms occurred in 2 (2.1%): very delayed device occlusion (1) and intraprocedural distal embolism (1). Eighteen patients (45%) showed improvement in pre-existing cranial nerve dysfunction because of the aneurysm’s mass effect, 3 patients (7.5%) worsened. One patient died of systemic organ failure unassociated with the procedure. Morbidity and mortality rates were 4.3% and 1.1%, respectively. PED embolization for unruptured large and giant ICA aneurysms is safe and efficacious. Physicians should be observant of characteristic risks associated with FD therapy.