With the development of technology, wireless sensor networks (WSNs) performing sensing and communication tasks will be widely deployed in the near future because they greatly extend our ability to monitor and control the physical environment and improve the accuracy of information gathering. Since the sensors are usually deployed in hostile environments and cannot get recharged frequently, the power in sensors is the scarcest resource. Recently many mobility control protocols have been put forward as an effective mechanism to minimize energy consumption. However, security issues in all these protocols have not been discussed so far. In this paper, we will address these issues and point out a new privacy issue (the sink location privacy). A privacy preserving scheme is proposed to protect the sink. Analysis of the scheme shows its effectiveness in sink protection. This scheme can be well integrated into most mobility control protocols to enhance their security.