Social function is an important concept in the mental and social health of older adults, and personality is a factor that drives many activities and communications. But their relationship is not explained. This study aims to determine the relationships between personality characteristics and social function in older adults. This is a cross-sectional descriptive-analytical study. We entered 550 eligible community-dwelling older adults through multi-stage cluster sampling in the study. The data were collected through the Big Five Inventory-44 (BFI-44) and Social Adaptation Self-evaluation Scale (SASS). Data were analyzed using SPSS software v.16 via t-test, ANOVA, Pearson’s correlation and multiple linear regression. There was a significant positive correlation between all personality characteristics and social function (p < .01). Age (β = -0.12), extraversion (β = 0.20), neuroticism (β = -0.11), openness to experience (β = 0.35), and conscientiousness (β = 0.16) are able to explain the variance of quality of activities, but agreeableness was ineffective. Also, quality of relationships variance is explained by age (β = -0.08), extraversion (β = 0.21), neuroticism (β = -0.12), openness to experience (β = 0.28), agreeableness (β = 0.11) and conscientiousness (β = 0.14). The changes in social function are predictable by BFI. Neuroticism has a negative effect on both the quality of relationships and the quality of activities, but agreeableness does not affect the quality of activities.