Privacy is one of the critical concerns that hinder the adoption of public cloud. For storage, encryption can be used to protect user's data. But for outsourced data processing, for example MapReduce, there is no satisfying solution. Users have to trust the cloud service providers totally. In this work, we propose PFC, a FPGA cloud for privacy preserving computation in the public cloud environment. PFC leverages the security feature of the existing FPGAs originally designed for bitstream IP protection and proxy re-encryption for preserving user data privacy. In PFC, cloud service providers are not necessarily trusted, and during outsourced computation, user's data is protected by a data encryption key only accessible by trusted FPGA devices. As an important application of cloud computing, we apply PFC to the popular MapReduce programming model and extend the FPGA based MapReduce pipeline with privacy protection capabilities. Proxy re-encryption is employed to support dynamic allocations of trusted FPGA devices as mappers and reducers. Finally, we conduct evaluation to demonstrate the effectiveness of PFC.