Somatosensory evoked potential is the reaction induced by stimulation the peripheral sensory nerve and conducted along the sensory pathway to the cortical sensory area. Wave P300 is a long-latency (about 300 msec), high amplitude positive component of event-related potentials. It represents the cognitive functions, including memory, calculation, and Concentration etc. When the cognitive functions decrease, a lengthening in latency and a decrease in amplitude of P300 wave is observed. The aim of this study is to evaluate the change of wave P300, which is induced by an electrical stimulus applied to the index lingers of the subjects, before, duriang and after acupuncture at Hegu (LI-4) points. The study design used in this study is ”within-group comparison”, which compares the values of P300 waves at different time periods among the same individuals. The results showed that during acupuncture latency increased significantly while amplitude at Pz and C3' decreased significantly, and that the levels were maintained for 20 minutes after removal of needles. Forty minutes after removal of needles, both latency and amplitude returned to pre-acupuncture levels.