This study examined effects of hedonic tones produced by odor stimuli on human mental performance. Thirty subjects were equally assigned to one of three olfactory stimulus groups: a control group with blank air, a Rose and a Ylang-ylang scented air group. From a preliminary study, we selected the latter as an unpleasant odor stimulus. The subjects of a different group received a different olfactory stimulus into a their nostril while they performed an audio-visual dual task. In this task the subjects were required to discriminate the difference in duration of a tone stimulus and to press a right hand key when they found the tone shorter. Engaging in this performance they had to attend to a red light rested in front of them and to press a left hand key when the light was turned off. For this visual performance the subjects of the Ylangylang group improved the detection of the signals in the first odor block. For the auditory performance, however, they made more errors than those of other groups. These results suggested that whiffs of an unpleasant odor would enhance detectability of the visual signal, but inhibit cognitive processes associated with discrimination in the dual task performance.