The experiment was conducted to investigate the effects of smiling expression on cognitive and emotional processes during the introduction of negative mood and cognition by self-focused attention. The mechanisms underlying such effects were also examined, with reference to Interacting Cognitive Subsystems framework (Teasdale & Barnard, 1993). We induced the self-focused attention for all 33 participants but the timing and type of facial expressions manipulated differed among three conditions: control condition (required to move the facial muscles which are unrelated with smiling), buffer condition (required to “smile” before the self-focused attention), and attenuation condition (required to “smile” after the self-focused attention). The results showed that the negative mood was increased in the control group while it was decreased in two experimental groups. Furthermore, the positive mood was decreased in the control group, and increased in the buffer group. The contents of spontaneous thought during experiment were more positive among two “smile” conditions than control condition. These results suggest the importance of smiling before and during negative self-focused attention.