The recent spectacular growth of volunteers in China after Wenchuan Earthquake has drawn the attention of researchers and policy makers. Most people regard it as an indicator of China's emerging civil society. This paper analyzes volunteering through both theoretical and empirical analyzes. It is argued that open institutions and groups are the link between volunteering and civil society. Institutional volunteering is less closely related to civic engagement in China. Data of volunteers who registered after Wenchuan Earthquake shows a quick decline over time, suggesting the existence of crowding out effect. Our empirical analysis uses the time to volunteer as the major variable for the supply of both individual volunteers and groups. The finding supports the private-good model of volunteers' motivation. The opportunity costs for volunteers, such as distance to the disaster area and skills, are important factors to volunteer supply.