Abstract: With the projects that governmental departments purchase social work service to mediate petition as the research object, this thesis attempts to reveal the operation mechanism and development dilemma of district-neighborhood petition mediation mechanism via social work service. We shall admit that professional social work organizations have achieved a benign interaction between the state and society in terms of compliant letters and visits mediation, activated national administration resources and enhanced efficiency of petition mediation by linking to national organization networks via a series of regulations. Therefore, it is an exploratory and effective attempt for the mediation mechanism which aims to maintain social stability to adopt professional social work. Of course, both the consignor (government) and the consignee (social organizations) need to understand correctly and objectively the role and status of professional social work in dealing with petition and orientate precisely social work’s involving in order to maintain its professionality and legitimacy. Moreover, we should not limit the role of professional social work to the social conflicts that are difficult for the department of petition to deal with, but put the professional social work in a bigger mediation mechanism, endow it with more freedom and trust, promote the development of the overall work of petition and eventually correct the malpractices in current petition system.