Anterior clinoidectomy is a key procedure during exposure of parasellar tumors and clipping of paraclinoid aneuryms. By doing anterior clinoidectomy, neurosurgeons gaining working length on vital structures like internal carotid artery and optic nerve, so that they can achieve proximal control on ICA, or mobilize optic nerve for tumor removal and optic nerve decompression. In this study, we measured the size of bony anterior clinoid process, then the length of ICA and optic nerve before and after anterior clinoidectomy. We found that, 50% more working length on ICA and 100% on optic nerve are available to neurosurgeons by doing extradural anterior clinoidectomy.