This paper investigates the influence of social networks and evaluation factors such as empowerment, subjective norms and cost on volunteers’ behavior and intentions. Volunteer group members in Ashiya City (Hyougo, Japan) and other nearby cities were surveyed. Volunteer group representatives distributed questionnaires to members of their group and related groups, to be returned by mail. A total of ninety-one participants responded. Structural Equation Modeling revealed that social networks, especially strong ties (intra and intergroup), had direct influence on members’ behavior, whereas evaluation factors mainly influenced members’ intentions. These results suggest that group members determine their behavior and intentions multilaterally, based on social networks and evaluation factors.