Web search engines are now using tens of thousands of index servers, which consume huge amount of power. This problem has motivated some research work into power efficient index serving. In this paper, we investigate FPGAs as the implementation platform. We propose the architecture of an FPGA-based hardware inverted index search engine. We present the system architecture and discuss implementations of essential components, including decoder, matcher and ranker. We successfully boot up the FPGA-based search engine and run experiments on real-world data from a commercial search engine. The targeted FPGA-based hardware index server could achieve up to 19.52X power efficiency and 7.17X price efficiency over an Intel Xeon server with highly optimized software. This is the first complete work using FPGAs to implement query processing for Web search engines.