Recently, botulinum toxin has been widely used for the management of spasticity. However it's mechanism of action in the skeletal muscle has not been well clarified. This study was performed to investigate the histopathologic changes in the skeletal muscle after botulinum toxin injection, and to determine the clinical standards of muscle fiber conduction study as an objective indicator for the changes of muscle fiber. As a study group, 35 Sprague Dawley rats were injected intra-muscularly with the botulinum toxin type A around two heads of right gastrocnemius muscle. After the injection of botulinum toxin, histopathologic studies and muscle fiber conduction studies were performed in 5 rats of the study group at 0, 1, 3, 5, 7, 14, and 28th day respectively. Based on the morphologic studies, the mechanisms of paralysis following the botulinum toxin injection were found to be both myogenic and neurogenic, and the motor function recovered through the formation of new motor end-plate and proliferation of Schwann cells. The muscle fiber conduction study revealed that the mean latencies of study group at 1, 3, 5, 7, and 14th day after the injection of botulinum toxin were significantly prolonged than those of the control group(p<0.05). The prolongation and slow recovery of latencies in a muscle fiber conduction study after the injection of botulinum toxin significantly reflect the morphologic changes of paralized skeletal muscles.