The main purpose of the present study was to examine mediating effects of affect variability on relationship between active participation in activities and depression of the elderly. A sample of 150 senior citizens in their 60s participated in a survey that asked about four types of participation activities (self-development activity, active social interaction, volunteering activity, leisure activity) and their depression level. Participants also recalled as many events as possible through a daily reconstruction method and their emotional experience felt for each event (e.g., excited, confident, happy, anxious, angry, peaceful, stable, depressed, and bored). Based on collected emotional data, spin, the index of affect, was calculated. Results indicated that higher levels of all social participation types predicted lower levels of depression. In terms of mediating effects, associations of depression with average value of participation activities, self-development activity, and active social interaction were mediated by affect variability. However, affect variability did not mediate the relationship of volunteering and leisure activities with depression. Results of this study showed that activities for self-improvement and interpersonal relationship could increase emotional stability and consequently decrease depression in the elderly.