According to the reform background of electricity market as well as the increasing flexible resources, an idea and market framework of generation-demand cooperative ancillary service is proposed in this paper. The flexible resources in the generation-side such as conventional thermal units, large hydropower, and pumped storage are dispatched cooperatively with the load-side flexible resources such as the smart residential loads, electric vehicles, storage, and distributed generators for improving the ancillary service level of power grid. The market trading framework of generation-demand cooperative ancillary service is built. As an example, the economic benefits of electric vehicles participating in frequency regulation service cooperated with the conventional thermal units are analyzed. The results indicate that the proposed idea and market framework can integrate the whole flexible resources for providing the ancillary service of power grid and has a good application prospect.