This paper investigates the factors affecting medical bloggers' knowledge sharing behavior from both personal and external aspects. We develop a model based on the social cognitive theory and augment it with the social capital theory. The model is empirically examined based on the survey data collected from 75 bloggers writing on medical issues, and evaluated with confirmatory factor analysis and structural equation modeling. Among the personal factors, we examined outcome expectations including reputation and enjoyment in helping others. Encouragement by others, identification and interaction ties were investigated as external factors. Enjoyment in helping others and reputation were found to have significant direct affect on medical bloggers' knowledge sharing behavior, while encouragement by others, identification, and interaction ties showed no significant direct affect. However, encouragement by others exhibited significant impact on reputation and enjoyment in helping others. The implications for theory and practice, and future possible research are discussed.