The relationship between sensory estimation and evoked potential when pleasant or unpleasant smell delivered to human nose was investigated. 10 healthy men participated. First, the subject was presented gamma-undecalactone (pleasant smell) or isovaleric acid (unpleasant smell), and instructed to estimate the odor magnitude and pleasantness/unpleasantness (sensory test session). Then, evoked potentials of the subject were measured from 19 scalp electrodes when pleasant or unpleasant smell were delivered 100 times to the subject, respectively (EEG measurement session). In the sensory test session, both the evaluation of odor magnitude and pleasantness/unpleasantness were significantly changed according to the concentration of smells. On the Pz scalp electrode, the positive potentials at the latency of 610 ms and 450 ms were observed in the pleasant and unpleasant stimulation, respectively. Statistically, it was found that the variance of the positive potential latency in unpleasant stimulation was significantly smaller than that in pleasant stimulation. It was also found that the positive potential latency in unpleasant stimulation was significantly earlier than that in pleasant stimulation. The small variance of latency and the earlier latency for unpleasant smell could be considered to reflect human behavior, such as quick reaction for avoiding dangerous odor to save one's life.