To enhance the accuracy for determining the precise localization, the findings of the compound nerve action potentials (CNAPs) of the common peroneal nerve (CPN) were investigated in patients with common peroneal mononeuropathy (CPM) in the knee, and the sural sensory nerve action potentials (SNAPs) were also analyzed. Twenty-five patients with CPM in the knee were retrospectively reviewed. The findings of the CNAPs of the CPN recorded at the fibular neck and the sural SNAPs were analyzed. The lesion was localized at the fibular head (abnormal CNAPs) and at or distal to the fibular head (normal CNAPs). Seven patients were diagnosed as having a lesion at or distal to the fibular neck, and 18 cases were diagnosed as having a fibular head lesion. The sural SNAPs were normal in all the cases of lesion at or distal to the fibular neck. Among 18 cases of fibular head lesion, the sural SNAPs were normal in 7 patients: two cases of conduction block and 5 cases of mild axon loss. Eleven patients showed abnormal sural SNAPs. Of those, 9 cases were severe axon loss lesions and 2 patients were diagnosed as having severe axon loss with conduction block. The recording of the CNAPs may enhance precise localization of CPM in the knee. Moreover, the sural SNAPs could be affected by severe axonal lesion at the fibular head.