Occipital artery (OA) to the posterior inferior cerebellar artery (PICA) bypass is indispensable for the management of complex aneurysms of the PICA that cannot be reconstructed with surgical clipping or coil embolization. Although OA-PICA bypass is a comparatively standard procedure, the bypass is difficult to perform in some cases because of the location and situation of the PICA. We describe the usefulness of the unilateral trans-cerebellomedullary fissure (CMF) approach for OA-PICA bypass. Thirty patients with aneurysms in the vertebral artery (VA) or PICA were treated using OA-PICA bypasses between 2010 and 2015. Among them, the unilateral trans-CMF approach was used for OA-PICA anastomosis in 13 patients. The surgical procedures performed on and the medical records of all the patients were retrospectively reviewed. The unilateral trans-CMF approach was performed for two reasons depending on the PICA location or situation: either because the caudal loop could not be used as a recipient artery because of arterial dissection (3 patients) or because the tonsillo-medullary segment that was located in the upper part of the CMF did not have a caudal loop that was large enough (10 patients). The trans-CMF approach provided a good operative field for the OA-PICA bypass and the anastomosis were successfully performed in all patients. When the recipient artery was located in the upper part of the CMF, the unilateral trans-cerebello-medullary fissure approach provided a sufficient operative field for OA-PICA anastomosis.