DescriptionAs the internet becomes more pervasive, people shift their self-expression and social interaction activities towards online platforms. It is therefore relevant to relate personality traits to behavior in social media domains. In this study, I investigated the relationship between one’s social media profiles and their MBTI personalities. I performed this research based on 4000 Twitter users who self-report their personalities and 425,752 tweets these users posted. I extracted 26 features that map into five categories of the user’s social media activities: Social Connections, Social Activities, Content Topic, Content Sentiment, and Temporal Patterns of Activities. I used two-sample t-tests and stepwise logistic regressions to examine the association between people’s social media activities and their personality traits. Then I used machine learning methods to predict one’s personality based on their social media profiles. Findings indicate that: 1) there are strong associations between personality and social media activities; 2) MBTI personality could be predicted by social media data and the best model achieved an average testing accuracy of 67.6%. I discuss the implications for further research in broader domains.