The asymmetry of alpha wave power in frontal regions of the brain has been considered to be associated with the state of mind (positive or negative affect). Substantial research has focused on the relations between emotion and EEG changes in the frontal area. The present study examined changes in alpha wave power in left and right frontal areas during the presentation of odor, and compared them with the results of comfort level evaluation by the Yoshida method. Eight subjects having no abnormalities in the sense of smell participated in the experiment. Subjects smelled the odor for five minutes with their eyes closed, and EEGs and heart rates were recorded before, during and after smelling the odor. EEGs recorded at Fp1 (left) and Fp2 (right) were used in the analysis. The results suggest that the state of mind in subjects who have greater left than right activation shifted more from “unpleasant” to “pleasant” mood while the opposite pattern was observed in another subjects with greater right frontal activation.